Geometric Design Guide for Canadian Roads Figure 2.1.4.12 Examples Of

Geometric Design Guide for Canadian Roads

Figure 2.1.4.12 Examples of Good and Poor Alignment Coordination and Aesthetics ...... 2.1.4.15 Figure 2.1.5.1 False Grading and Cross-Slopes ...... 2.1.5.3 Figure 2.1.5.2 Application of Cross-Slope on Various Types of Roads ...... 2.1.5.4 Figure 2.1.5.3 Maximum Algebraic Difference in Pavement Cross-Slope of Adjacent Traffic Lanes ...... 2.1.5.7 Figure 2.1.6.1 Schematic of Basic Number of Lanes ...... 2.1.6.1 Figure 2.1.6.2 Typical Examples of Lane Balance...... 2.1.6.3 Figure 2.1.6.3 Coordination of Lane Balance and Basic Lanes ...... 2.1.6.4 Figure 2.1.6.4 Typical Express Collector System ...... 2.1.6.6 Figure 2.1.7.1 Illustration of Route Continuity ...... 2.1.7.2 Figure 2.1.7.2 Examples of Lane Continuity ...... 2.1.7.3 Figure 2.1.7.3 Types of Weaving Sections ...... 2.1.7.5 Figure 2.1.7.4 Solutions for Undesirable Weaving ...... 2.1.7.6 Figure 2.1.7.5 Method of Measuring Weaving Lengths ...... 2.1.7.7 Figure 2.1.8.1 Collision Involvement Rate for Trucks ...... 2.1.8.2 Figure 2.1.8.2 Performance Curves For Heavy Trucks, 120 g/W, Decelerations & Accelerations ...... 2.1.8.4 Figure 2.1.8.3 Performance Curves for Heavy Trucks, 180 g/W, Decelerations & Accelerations ...... 2.1.8.5 Figure 2.1.8.4 Performance Curves for Heavy Trucks, 200 g/W, Decelerations & Accelerations ...... 2.1.8.6 Figure 2.1.8.5 Climbing Lanes Overlapping on Crest Curve ...... 2.1.8.10 Figure 2.1.8.6 Climbing Lane Design Example ...... 2.1.8.11 Figure 2.1.9.1 Typical Passing Lane Layout ...... 2.1.9.2 Figure 2.1.9.2 Typical Traffic Simulation Output of Percent Following and Correlation with Level of Service ...... 2.1.9.4 Figure 2.1.9.3 Alternative Configurations for Passing Lanes ...... 2.1.9.5 Figure 2.1.10.1 Forces Acting on Vehicle in Motion...... 2.1.10.2 Figure 2.1.10.2 Basic Types of Designs of Truck Escape Ramps ...... 2.1.10.4 Figure 2.1.10.3 Typical Layout of Truck Escape Ramp ...... 2.1.10.8 Figure 2.1.10.4 Typical Cross Section of Truck Escape Ramp ...... 2.1.10.8

December 2009 Page 2.1.v Alignment and Lane Configuration

Page 2.1.iv September 1999 Geometric Design Guide for Canadian Roads utilized beyond which the lateral friction is kept generally accustomed to using more lateral constant and superelevation is increased rapidly friction when manoeuvring through to maximum. This form of distribution is referred intersections. to as “Method 2” in AASHTO and is used in some urban areas. This method is particularly Urban Roadways: Design Domain advantageous on low speed urban streets Quantitative Aids where, because of various constraints, superelevation frequently cannot be provided. Figure 2.1.2.5 illustrates an alternative method for urban use for applying superelevation over Rural & High Speed Urban Applications: a range of radii. This method, for urban Design Domain Quantitative Aids conditions, is based on retaining normal crown until a lateral friction factor of 0.05 is required, Based on the described distribution method at which point reverse crown is introduced. This (Method 5), the recommended superelevation friction factor is approximately equivalent to rate for various radii from 7000 m to the roadway icing conditions. An assumed minimum radius, for each design speed and for operating speed of 10 km/h above design speed superelevation rates of 0.04 m/m, 0.06 m/m, is used in the derivations to introduce the safety and 0.08 m/m have been developed. Also factor for overdriving or adverse pavement included are the spiral parameters applicable conditions. Figure 2.1.2.5 illustrates the to rural and high speed urban roadways (spiral derivation of these values for a design speed parameters will be addressed later in this of 50 km/h, and maximum superelevation Section). These values are illustrated in values of 0.04 m/m and 0.06 m/m. Tables 2.1.2.5 to 2.1.2.7 and are based on relationships presented in a 1974 publication31. For this alternative design method the minimum radius in reverse crown is determined by Urban Roadways: Design Domain distributing the lateral friction coefficients in a Technical Foundation rate proportional to the inverse of the radius. The linear distribution occurs between the Tables 2.1.2.5 to 2.1.2.7 indicating the amount lateral friction coefficient at the inverse of the of superelevation for various radii are based on minimum radius for normal crown, calculated relatively low lateral friction values which as noted above, and the maximum lateral give high superelevation rates. These friction coefficient of the inverse of the minimum superelevation rates and spiral parameters are radius corresponding to full superelevation. appropriate for rural roadways and higher speed Because a unique minimum radius corresponds urban roadways where intersections are widely to each of the two full superelevation rates, spaced and access is restricted. For other urban +0.04 m/m and +0.06 m/m, the proportional situations higher rates of superelevation are distribution produces different values for often not attainable due to constraints such as minimum radius at reverse crown. access requirements, elevations of the adjacent properties, drainage considerations, the profiles This alternative method provides superelevation of intersecting streets and limiting slopes on rates between the rural and urban high speed boulevards and sidewalks. values, and the low speed urban values described earlier. Table 2.1.2.4, Table 2.1.2.8 An alternative method for selecting and Table 2.1.2.9 reflect this alternative design superelevation rates under low speed urban method for selecting superelevations over a design conditions is to use higher lateral friction range of radii from 20 m to 7000 m. This values, thereby reducing the superelevation provides the designer with a consistent means requirements. These superelevation rates are of selecting superelevation applicable to most illustrated in Figure 2.1.2.4 and generally apply urban conditions. Table 2.1.2.4 was discussed to roadways through intersection areas where earlier under minimum radius, and Tables design constraints are numerous and limiting 2.1.2.8 and 2.1.2.9 provide superelevation rates superelevation offers operational advantages for varying speeds and radii using the urban for turning or crossing traffic. Drivers are method described above. Table 2.1.2.8 is for

September 1999 Page 2.1.2.11 Alignment and Lane Configuration lane lane lane lane / Radius 1 2 is spiral parameter in metres is superelevation below the dashed line, use 4 lane values x 1.15. For 6 lane pavement: above the dashed line use 4 lane values, A divided road havingtreated a median as less a single than pavement. 3 m wide may be e A NC is normal cross section RC is remove adverse crownSpiral and length, superelevate L at = normal A2Spiral rate / parameters Radius are minimum and higher values may be used is spiral parameter in metres is superelevation, m/m = 0.04 m/m • • • • • • • • •

e NC is normal cross section RC is remove adverse crown and superelevate at normal rate Spiral length, L = A Spiral parameters are minimum and higher values by be used For 6 lane pavement: above the dashed line use 4 values, A divided road having a median less than 3.0 m wide may be max Notes: • • • • • • • • below the dashed line, use 4 lane values x 1.15. treated as a single pavement. =0.04

max

e AAAAAAA A A 24 e 24 e24e 24 e 24 e 2 4 e 2 4 lane lane lane lane lane lane lane lane lane lane 40 50 60 70 80 90 100 min R = 60 e 0.040 50 50 908070 0.03660 0.038 50 0.040 50 0.040 50 50 50 50 0.040 50 70 50 min R = 100 70 (m) 900800700 NC600 NC500 NC400 NC350 NC300 RC250 RC220 95 RC200 NC 90 RC180 NC 95 80 RC160 RC 0.021 90 75 RC140 0.023 RC 80 70 65 140 0.020120 RC 0.025 75 130 65 0.023 105100 140 100 0.028 70 65 120 60 0.026 130 0.031 105 90 100 65 55 RC 0.029 0.031 120 0.034 85 60 RC 0.025 50 RC 0.027 0.033 90 80 0.021 150 75 55 RC 50 115 0.035 110 175 85 140 70 55 130 150 0.031 115 0.037 80 165 110 75 175 70 50 140 130 0.020 0.034 100 0.039 0.031 70 165 0.033 70 0.023 0.036 0.038 0.026 RC 165 0.040 100 90 70 125 70 120 0.039 RC 155 140 90 90 0.036 70 165 70 190 90 125 0.040 120 70 90 155 110 140 180 90 90 0.040 0.026 190 0.038 90 70 0.037 0.039 110 90 0.029 180 0.033 0.039 0.040 90 175 110 0.021 135 90 135 min 0.040 165 110 0.040 R 110 150 0.023 = 175 110 150 90 200 135 135 110 165 110 135 110 150 190 0.032 0.040 200 0.040 min 110 0.040 R 135 = 0.035 190 0.038 200 185 135 0.027 160 160 175 160 0.029 min R 135 185 210 = 280 160 160 175 200 160 min R 0.036 210 = 380 0.040 0.039 200 0.040 200 0.032 190 190 min 190 R 0.034 = 200 225 490 190 190 210 190 225 210 Notes: 700050004000 NC3000 NC2000 NC1500 NC1200 NC1000 NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC RC NC NC 180 NC 180 NC NC RC RC 200 RC 240 200 215 240 NC 0.020 215 NC NC RC 210 NC RC 210 RC 260 230 0.025 260 300 230 225 300 RC 0.021 NC 225 NC RC 275 245 0.030 NC 275 245 NC 235 315 0.022 0.026 235 315 290 260 RC 290 260 NC 335 NC 335 NC RC 410 410 Speed (km/h) Radius Design Table 2.1.2.5 Table Parameters, e Superelevation and Minimum Spiral

Page 2.1.2.12 December 2009 Geometric Design Guide for Canadian Roads 1 0.022 / Radius m/m 2 = 0.06 m/m 0.048 0.051 0.026 0.032 0.038 0.043 0.045 max is spiral parameter in metres is superelevation 390 300 270

e A NC is normal cross section RC is remove adverse crownSpiral and length, superelevate L at = normal A Spiral rate parameters are minimum andFor higher 6 values lane may pavement: be above used below the the dashed dashed line line, use useA 4 4 divided lane lane road values, values having xas a 1.15. median a less single than pavement. 3.0 m wide may be treated • • • • • • • • • Notes: =0.06

max

e lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane e AAAAAAAAAA 40 50 60 70 80 90 100 110 120 130 min R = 55 0.059 50 60 908070 0.051 5060 0.054 60 50 0.060 0.056 60 65 50 0.059 0.060 60 70 50 65 60 70 min R = 90 (m) lane lan 900800700 NC600 NC500 NC400 NC350 120 RC 120300 0.023 0.024 RC 100 125250 100 90 0.025 RC 0.027 150 125 90 0.021 120 90220 150 0.028 0.030 150 0.031 140 0.023 120 140 90200 150 100 80 0.031 0.034 175 140 0.034 140 0.025 100 0.027 125 80 100 75 175180 0.037 0.034 160 0.038 0.037 150 0.029 125 100 150 80160 160 115 70 0.036 0.041 180 150 90 0.041 0.040 150 0.031 120 0.034140 140 110 80 180 70 0.042 175 0.038 100 85 0.045 175 0.043 0.034 150 115 175 0.044 75 175 125120 60 0.040 0.046 200 175 0.048 80 0.045 90 100 175 0.036 135 0.039 150 120 200 75 60 0.043 0.048100 175 110 0.048 75 0.051 175 0.047 90 0.039 160 125 175 0.051 75 175 60 135 90 0.052 200 175 0.054 0.046 0.050 70 0.049 90 120 185 0.042 150 0.045 160 70 125 200 100 0.054 200 125 60 90 0.052 70 0.057 0.052 0.049 185 90 0.043 175 140 0.055 190 0.057 200 160 65 85 0.059 100 225 195 65 0.059 110 50 0.054 125 200 85 0.046 0.055 165 0.057 0.050 190 160 225 120 0.060 100 210 65 135 85 0.056 0.060 80 110 200 65 0.051 190 160 0.060 220 0.059 0.058 0.060 215 190 85 0.060 110 235 210 0.059 125 110 90 160 220 0.054 65 75 190 0.060 0.058 220 250 125 85 110 min 220 160 0.060 90 R 125 220 0.060 0.057 220 = 70 440 110 240 min 125 250 235 R min 90 = R 0.059 min min 110 750 = 0.060 R R 600 270 = = min 250 340 250 R 250 min 0.060 = R 190 260 = 280 260 130 300 min R = 950 70005000NCNCNCNCNCNCNCRC555555RC580580RC600600 4000 NC30002000 NC1500 NC1200 NC1000 NC NC NC NC NC NC NC NC NC NC RC NC 170 NC 170 0.021 NC 175 RC 175 NC 0.027 RC 225 200 225 200 200 NC 200 RC 0.032 0.023 200 NC 250 225 200 RC 250 225 0.037 0.024 0.028 225 275 250 225 225 NC 275 250 225 0.040 0.029 0.033 RC 235 270 240 NC 235 300 275 240 0.048 NC 300 0.029 0.034 245 0.023 290 260 255 300 290 260 0.054 350 0.036 0.043 260 NC 0.022 305 270 280 335 305 270 0.058 335 0.042 0.049 280 NC 0.029 315 285 300 350 315 RC 290 350 0.049 0.055 390 0.034 330 295 400 365 335 320 365 NC RC 0.040 380 410 380 RC 410 0.020 475 430 475 430 0.024 RC 450 NC 495 450 495 0.036 465 RC 465 515 515 0.023 540 RC 540 RC 710 710 Radius e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 Design Speed (km/h) Table 2.1.2.6 Table Parameters, e Superelevation and Minimum Spiral

DecemberSeptember 2009 1999 Page 2.1.2.13 Alignment and Lane Configuration / Radius 2 / Radius 2 1 is spiral parameter in metres is superelevation, m/m A e NC is normal cross section RC is remove adverse crown and superelevate at normal rate Spiral length, L = A Spiral parameters are minimum and higher values by be used For 6 lane pavement: above the dashed line use 4 values, A divided road having a median less than 3.0 m wide may be is spiral parameter in metres is superelevation below the dashed line, use 4 lane values x 1.15. treated as a single pavement. Notes: • • • • • • • • = 0.08 m/m treated as a single pavement.

e A NC is normal cross section RC is remove adverse crown andSpiral superelevate length, at L normal = rate A Spiral parameters are minimum andFor higher 6 values lane may pavement: be above used thebelow dashed the line dashed use line, 4 useA lane 4 divided values, lane road values having x a 1.15. median less than 3.0 m wide may be max Notes: • • • • • • • • •

max lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane lane AAAAAAAAAA e an 40 50 60 70 80 90 100 110 120 130 l min R = 50 0.080 50 60 908070 0.064 5560 0.067 70 5550 0.077 0.071 65 50 65 0.080 0.075 60 50 65 80 0.080 0.080 60 50 75 65 min 60 R = 120 75 min R = 80 (m) 900800700 NC600 NC500 NC400 RC350 0.021 120 100 120300 100 0.025 RC 0.026 0.030 90 0.020250 125 120 150 0.028 150 90 125 90 120 0.023220 150 150 0.031 0.034 0.035 0.039 140 0.025 90 0.027 80 140200 100 125 175 140 160 0.035 0.038 140 90 110 135180 0.030 75 175 160 100 0.042 0.045 0.048 150 0.039 0.042 0.032 0.035 150 85 105160 115 140 70 180 150 175 90 0.041 0.049 160 0.047 125 150 0.038 180140 80 70 175 110 0.050 100 0.054 0.044 0.056 165 85 0.039 0.042 165 0.051 80 125 0.053 65 125 150120 200 165 175 100 0.058 0.047 185 100 80 150 0.054 175 0.046 80 200 195 0.059 65 120 0.058 120 0.063100 0.064 175 0.051 0.046 80 100 0.049 0.057 100 175 150 0.063 135 80 160 65 200 190 0.062 200 120 0.067 200 75 120 0.055 100 165 200 0.053 0.060 220 75 220 0.069 95 125 0.067 60 0.065 140 0.071 0.073 185 0.051 0.055 75 110 190 95 0.064 160 0.061 0.072 160 190 90 75 110 225 200 210 135 0.075 225 55 125 0.068 70 0.073 185 215 0.061 240 0.069 90 235 110 0.078 160 150 110 0.080 0.77 200 70 90 0.062 0.80 0.066 0.075 125 70 0.072 175 90 0.080 190 125 235 230 220 220 0.074 110 150 220 105 0.080 160 85 0.080 200 0.076 0.072 275 250 85 270 125 0.080 0.078 190 65 250 175 125 220 0.071 0.080 85 100 0.075 160 110 0.080 200 150 250 260 250 0.080 250 min 80 175 120 R 100 min 85 = min 0.079 R min 110 300 285 530 R 295 = R 0.080 = 230 = 250 670 0.078 390 120 min 0.080 R 95 85 = 280 min 285 280 300 R min = R 0.080 170 330 = 330 830 95 280 300 700050004000 NC3000 NC2000 NC1500 NC1200 NC1000 NC NC NC NC NC NC NC NC NC NC NC NC RC NC 170 NC 170 0.023 NC 175 RC 175 0.029 NC RC 225 200 NC 225 200 200 0.021 0.036 NC 200 255 200 0.026 NC 260 220 200 0.027 RC 0.043 225 250 225 0.032 270 250 225 225 0.032 275 NC 0.047 225 270 0.021 240 0.038 NC 300 275 240 240 0.033 NC 300 0.057 240 290 0.026 250 NC 0.040 300 290 260 280 0.042 300 0.066 260 305 0.026 260 0.050 335 305 270 310 NC 0.050 335 0.074 285 315 0.033 NC 280 0.059 350 330 285 340 NC 0.058 350 320 330 0.040 RC 0.067 365 365 295 390 365 350 400 0.047 380 NC RC 380 RC 410 410 RC 0.023 480 430 480 430 0.028 RC 450 RC 495 450 0.033 495 RC 465 0.021 555 515 465 555 515 0.026 RC 540 580 540 580 RC 0.021 600 600 RC 710 710 Radius e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 e 2 3&4 Design (Speed km/h) Table 2.1.2.7 Table Parameters, e Superelevation and Minimum Spiral

Page 2.1.2.14 December 2009 Geometric Design Guide for Canadian Roads

9 Table 2.1.2.9 Superelevation Rate for Urban Design, emax= 0.06 m/m

Radius Design Speed (km/h) (m)30405060708090100 7000 NC NC NC NC NC NC NC NC 5000 4000 NC 3000 NC NC RC 2000 NC RC RC 1500 NC RC 1200 RC 1000 900 NC 800 RC RC 700 NC 0.025 600 RC RC 0.035 500 NC RC 0.030 0.048 400 RC 0.026 0.045 Min R=440 350 RC 0.035 0.056 300 0.025 0.045 Min R=340 250 RC 0.036 Min R=250 200 0.024 0.053 180 0.030 Min R=190 160 0.037 140 RC 0.046 120 0.026 Min R=120 100 0.036 90 0.043 80 RC 0.052 70 0.024 Min R=75 emax = 0.06 m/m 60 0.032 NC = normal crown (-0.02 m/m) 50 0.044 RC = reverse crown (+0.02 m/m) 40 RC Min R=40 30 0.030 20 0.056 Min R=20 min. radius for normal 420 660 950 1290 1680 2130 2620 3180 crown min. radius for reverse 40 80 135 220 330 450 600 770 crown

September 1999 Page 2.1.2.19 Alignment and Lane Configuration

On tight horizontal curves, stopping sight Because of the variance, providing specific distances may need to be increased to adjustments for radii other than minimum is compensate for increased braking distances difficult. Suggested approximate adjustments caused by lateral friction effects. are:

Design Domain Quantitative Aids: • for the lower range of assumed operating Stopping Sight Distance on Horizontal speeds from 40 km/h to 80 km/h and for Curves horizontal curves not exceeding 110% of the minimum radius curve, the stopping Table 2.1.2.10 shows the stopping sight sight distance in Table 1.2.5.3 should be distance while braking on a minimum radius increased by 3.0% curve. Values are provided for the higher and lower limits for each of the three values of • for the higher range of assumed operating maximum superelevation. Also included for speeds from 40 km/h to 120 km/h and for comparison purposes are the normal stopping horizontal curves not exceeding 110% of sight distances from Table 1.2.5.3. the minimum radius curve, the stopping sight distance in Table 1.2.5.3 should be The results of the calculations for stopping sight increased by 5.5% distance on minimum radius curves indicates that there is a large variance in the amount of If adjustments that are more precise than increase for the various conditions. outlined above are required, the designer should use the formulae provided in this Section. 1. The increase in stopping sight distance based on the lower range of assumed 2.1.2.3 Spiral Curves operating speeds (left columns in Table 2.1.2.10) vary from 0.3% to 4.2%. Introduction

2. The increase in stopping sight distance A spiral curve is a curve with a constantly varying based on the upper range of assumed radius. The purpose of a spiral curve is to operating speeds (right columns in Table provide smooth transition and a natural driving 2.1.2.10) vary from 3.2% to 8.8%. path between a tangent and a circular curve.

Table 2.1.2.10 Calculated Stopping Sight Distance on Minimum Radius Curves

Design Assumed Normal Stopping Calculated Stopping Sign Distance on Speed Operating Sight Distance Minimum Radius Curves (m) (km/h) Speed (km/h) Table 1.2.5.3 (m) e = 0.04 e = 0.06 e = 0.08

40 40 45 46 46 46 50 47-50 60-65 60-66 60-66 60-66 60 55-60 75-85 77-90 77-90 77-90 70 63-70 95-110 98-120 97-120 97-120 80 70-80 115-140 117-152 116-152 116-151 90 77-90 130-170 135-180 134-180 134-181 100 85-100 160-210 161-219 160-218 160-219 110 91-110 180-250 181-259 181-259 120 98-120 200-290 205-302 204-301 130 105-130 230-330 229-338 229-338

Page 2.1.2.20 December 2009 Geometric Design Guide for Canadian Roads

Because spiral curves provide a natural path for radius of curve at a distance L from the beginning the motorist to follow, centrifugal forces increase of spiral ( Rα 1/L ). and decrease gradually as the vehicle enters and leaves the circular portion of the curve. This A2 = RL (2.1.8) minimizes encroachment upon adjoining traffic lanes, promotes speed uniformity, and increases safety. Where A is a constant called the spiral parameter and has units of length. A spiral curve provides a convenient and desirable arrangement for developing superelevation runoff. All clothoid spirals are the same shape and vary A change from normal crown to a fully only in their size. The spiral parameter is a superelevated cross section is applied along the measure of the flatness of the spiral, the larger spiral curve length. Where the pavement section the parameter the flatter the spiral. is widened around a circular curve, the spiral facilitates the transition width. Spiral curves also A spiral curve in which one end of the spiral enhance the roadway appearance because there has a radius of infinity is referred to as a simple are no noticeable breaks in the alignment. spiral and one in which the radii at both ends are less than infinity is referred to as a On rural roads, spiral curves should be utilized segmental spiral. An example of a segmental on new construction if the circular curves are spiral is a spiral between two circular curves of superelevated. Some exceptions can be made different radii but in the same direction. A simple with curves substantially flatter than the spiral is designated by its parameter and its end minimum required for the design speed. Also, radius. A segmental spiral is designated by its when appropriate, simple curves may be used parameter and its two end radii. on existing paved roads to avoid the need to make minor realignments. The Spiral Parameter: Technical Foundation

On urban roads, the incorporation of spiral Spiral parameters can be developed based on curves into horizontal main line alignment three criteria: design is normally limited to major arterials, expressways and freeways, where higher • the maximum permissible rate of change design speeds are used. Spiral curves are of centripetal acceleration (which typically only applied to roadways with design expresses comfort) speeds of 70 km/h and higher, and where superelevation of the circular curves is • superelevation runoff desirable. Spiral curves are also used in the design of interchange ramps. Where these • aesthetics roadway facilities have curbs, the use of spiral curves allows vehicles to negotiate the Figure 2.1.2.7 illustrates the conceptual results alignment at a constant offset from the curb, of these means of deriving the spiral parameter, thus increasing driver comfort. For low speed and the basis for each of these methodologies urban design and retrofit situations, spiral is discussed in depth following this Figure. curves are not typically used. The minimum spiral requirement for design is Overview: Technical Foundation the highest of the three values and for the smaller radii the comfort criterion controls, for The spiral form most commonly used for road the next larger set of radii the superelevation design is the clothoid, which, expressed runoff criterion controls, and for the larger radii mathematically, has the relationship where R the aesthetic criterion controls. varies with the reciprocal of L, where R is the

DecemberSeptember 2009 1999 Page 2.1.2.21 Alignment and Lane Configuration

Figure 2.1.2.7 Basis for Spiral Parameter Design Values1

Spiral Parameter Based on Comfort v 2 Centripetal acceleration is: R A vehicle travelling along a circular curve Where R is the radius (m) experiences centripetal acceleration. As the v is the speed (m/s) vehicle is moving from a tangent direction to a circular curve path, the radius of curvature Rate of change of centripetal acceleration is: decreases from infinity to that of the circular curve. During this time the centripetal v 2 acceleration is increasing from zero to a c = (2.1.9) constant. The rate of change of acceleration is Rt high if the transition length is short and low if it is long. The rate of change of centripetal Where t is travel time (s) acceleration is a measure of discomfort to the L t = vehicle occupants. If the transition length is v (2.1.10) short the vehicle occupants will experience a jerk. Lower values of acceleration and Where L is length of curve (m) corresponding long transition are preferred. v 3 c = (2.1.11) One of the purposes of the spiral is to provide a A 2 length over which the driver can effect a change in curvature of the travelled path so as to v1.5 A = (2.1.12) position the vehicle centrally within the lane. For c 0.5 comfort therefore the spiral curve should provide sufficient length to allow a smooth increase in radial acceleration.

Page 2.1.2.22 September 1999 Geometric Design Guide for Canadian Roads

If A is stated in metres, speed v in kilometres Table 2.1.2.11 Maximum Super- per hour and c in metres per second cubed, the elevation Runoff expression becomes: Between Outer 0.1464V 1.5 Edge of Pavement A = (2.1.13) c 0.5 and Centreline of Two-Lane Roadways Tolerable rate of change of centripetal acceleration varies between drivers. As a basis Design Speed Superelevation Runoff of design, the value used to provide minimum (km/h) (%) acceptable comfort is 0.6 m/s3. The expression then becomes: 40 0.70 50 0.65 1.5 0.1464V 60 0.60 A = (2.1.14) 0.60.5 70 0.55 80 0.51 A = 0.189V1.5 (2.1.15) 90 0.47 100 0.44 110 0.41 Using the above expression, the minimum spiral 120 0.38 parameter based on comfort can be calculated 130 0.36 for each design speed. It may be noted that the spiral parameter is independent of the radius. This is illustrated in Figure 2.1.2.7 by the comfort For a given speed and radius, superelevation line parallel to the abscissa. and superelevation runoff are known and minimum lengths can be calculated. From Spiral Parameter Based on minimum length and radius, the minimum spiral Superelevation Runoff parameter can be calculated, using the expression: For superelevation runoff, is defined as the slope A2 = RL (2.1.17) of the outer edge of a pavement in relation to the profile control line. The maximum permissible value varies with design speed, and is shown in Spiral Parameter Based on Aesthetics Table 2.1.2.11. Short spiral transition curves are visually The minimum length of transition, l , is given by unpleasant. It is generally accepted that the the equation length of a transition curve should be such that 100 we the driving time is at least 2 s. For a given radius l = 2s and speed, therefore, the minimum length and (2.1.16) minimum spiral parameter can be calculated using the expression: Where w = the width of pavement in 2 metres A = 0.56RV (2.1.18)

e = the superelevation being Spiral Parameter: Design Domain Quantitative developed in metres per metre Aids

s = the superelevation runoff, Quantitative expressions of the design domain percentage for the spiral parameter are given in Tables 2.1.2.5, 2.1.2.6 and 2.1.2.7 in l=measured in metres Subsection 2.1.2.2 for a range of design speeds. Designers should note the following application heuristics in using these tables.

December 2009 Page 2.1.2.23 Alignment and Lane Configuration

1. For any particular design speed and radius, radii is not too large. See Subsection 2.1.2.6 the highest value of spiral parameter as for additional guidance in this regard. determined by the methodologies discussed in the previous section are used 5. Circular, non-successive curves in the same for these calculations. direction joined by a short length of tangent should normally be joined instead by a spiral. 2. The design values in the tables are The use of tangents to achieve such minimum and higher values should be used connections results in what is commonly wherever possible in the interests of safety, referred to as a “broken back” curve. It is comfort and aesthetics. only justified when some other consideration, for example, property 3. In design it is convenient to select values constraints or construction cost, outweighs so that the radius is a standard value and the visual disadvantages. The term “short A/R is a rational number as this permits tangent” is very subjective and there are spiral properties to be read directly from several definitions for this term. One tables33. suggests that a short tangent length may be regarded as one which allows a driver Spiral Parameter: Design Domain Application on the first curve to see at least some part Heuristics of the following curve. Another definition suggests a broken back curve occurs when The application of spiral curves in horizontal the tangent length (m) is less than four alignment is a complex design problem that has times the design speed. If possible a more many variations. A number of design domain desirable solution to a broken back curve application heuristics dealing with some of the is to eliminate the short tangent and insert more common instances are provided below. an appropriate circular curve or better yet a segmental spiral curve. 1. A circular curve with simple spirals at both ends each having the same parameter 6. A change of direction from one tangent to value is referred to as a symmetrical curve. another may be accomplished by This condition represents the most successive spiral curves without a length of common practice for spiraled curves. circular curve between them. Such a transition curve is referred to as symmetrical 2. Unsymmetrical curves are common at where the two spiral parameters are the interchange ramps and loops and represent same, and unsymmetrical where they are the case noted in #1 above, but with different. The minimum permissible spiral different parameter values for the spirals parameter to be used in such a situation is at each end. the minimum for the design as shown in Tables 2.1.2.5, 2.1.2.6 and 2.1.2.7. 3. Successive circular curves with different radii but in the same direction are best joined by 7. A reversal in curvature direction may be a spiral curve. Where this occurs, the “joining accomplished through successive simple spiral” is referred to as a segmental spiral. spiral curves without a length of tangent The minimum spiral parameter to be used between them. The spiral parameters should is found by referring to Tables 2.1.2.5, 2.1.2.6 be at least the minimum for the design and 2.1.2.7 and using the smaller of the two speed. However, the alignment will have an radii. improved appearance if the minimum spiral parameter values are exceeded or if a 4. In some instances, successive circular length of tangent is inserted between the curves with different radii may connect two spirals. Superelevation is applied as directly to one another if the difference in

Page 2.1.2.24 June 2009 Geometric Design Guide for Canadian Roads

To illustrate the use of equations 2.1.20 and year. Additional safety benefits may accrue to 2.1.21, consider a curve with R = 0.35 km that the curves connected to the other ends of the is between tangents that are 0.9 km long and tangents. which intersect at a deflection angle of 120o (2.09 radians). The roadway is 11.2 m wide and There is a simple approximate way of determining has spiral transition curves. What would be the the collision reduction which is due to increasing safety consequences of increasing the radius the radius. Disregarding the safety benefit due 2 to 0.437 km . to shorter tangents, when R1 < R2:

In equation 2.1.20, the parameter 0.96 applies expected collision reduction per unit of time = to collisions/million vehicle km on a straight (collisions on 1 km of straight road per unit of section. Therefore, on the two tangents we time) x (reduction in path length in km) + 0.0245 expect 2 x 0.9 x 0.96 = 1.73 collisions/million x million vehicles per unit of time x (1/R2-1/R1) vehicles. The length of the curve is 0.35 x 2.09 = 0.732 km. On the curve one may expect (0.96 The “reduction in path length in km” is given by x 0.732 + 0.0245/0.35 - 0.012) x 0.97837-30 = 0.65 collisions/million vehicles. The correction factor (R -R ) x [2tan(D/2)-D] (2.1.22) for tangent length is e-(0.62 - 1.2x0.35) x (1.2 - 0.9) = 0.94. 1 2 Thus, with a 0.35 km radius curve one should expect 1.73 + 0.65 x 0.94 = 2.34 collisions/ in which D is the deflection angle in radians. million vehicles. Numerical Example: Suppose now that a curve with R = 0.437 km is

As in the previous numerical example, let R1 = considered for the same conditions. The larger o 0.437 km, R2 = 0.35 km, D = 120 (2.09 radians) radius implies a longer curve and shorter -6 tangents. Each tangent length is reduced by and Volume = 5000 x 365 x 10 = 1.825 million (0.437-0.35) x tan (2.09/2) = 0.15 km. Thus, on vehicles/year. The reduction in path length is the two tangents one expects 2 x (0.9 - 0.15) x (0.437-0.35) x [2tan (2.09/2) - 2.09] = 0.12 km. 0.96 = 1.44 collisions/million vehicles. The If there are 0.96 collisions/million vehicle km on length of the curve is now 0.437 x 2.09 = a straight road, then, with 1.825 million vehicles 0.913 km and with long tangents one may per year, one expects 0.96 x 1.825 = 1.75 expect on it (0.96 x 0.913 + 0.0245/0.437 - collisions/year for a kilometre of road. Thus, the 0.012) x 0.97837-30 = 0.8 collisions/million expected collision reduction is 1.75 x 0.12 + vehicles. The correction factor now is 0.0245 x 1.825 x (1/0.35-1/0.437) = 0.21 + 0.03 e-(0.62 – 1.2 x 0.437) x (1.2 - 0.75) = 0.96. Thus, with a = 0.24 collisions/year. R=0.437 km curve one should expect 1.44 + 0.8 x 0.96 = 2.21 collisions/million vehicles. If In summary, when a horizontal curve is to be AADT = 5000, the collision reduction is fitted between tangents with a given deflection (2.34 - 2.21) x 5000 x 365 x 10-6 = 0.24 collisions/ angle, the greater the radius of the curve the fewer collisions will occur on the roadway.

December 2009 Page 2.1.2.45 Alignment and Lane Configuration

Page 2.1.2.46 September 1999 Geometric Design Guide for Canadian Roads

3. Where possible, gradients lower than the Minimum Grades: Design Domain Application maximum values shown should be used. Heuristics

4. Maximum values should only be exceeded Rural Roadways after a careful assessment of safety, cost, property and environmental implications. 1. On uncurbed roadways, level grades are generally acceptable provided the roadway 5. The choice of maximum gradient may have is adequately crowned, snow does not a bearing on related design features; for interfere with surface drainage, and ditches example, whether or not a truck climbing have positive drainage. Roadway crown is lane or escape lane is required. discussed in Section 2.1.5. Refer to Chapter 2.2 for guidelines for the design of 6. While Table 2.1.3.1 provides general roadside open ditches and to relevant guidance, the designer should be aware drainage publications. that the factors that should be considered in establishing the maximum grade for a Curbed Roadways section of roadway include: (generally in urban areas)

• road classification 1. To ensure adequate drainage, curbed roadways typically have a minimum • traffic operation longitudinal grade of 0.50% or 0.60%, depending on local policy. • terrain 2. In certain rare design cases, when no other • climatic conditions alternative is feasible, a grade of 0.30% may be used as an absolute minimum • length of grade preferably in combination with highly stable soils and rigid pavements. • costs 3. For retrofit projects, longitudinal grades • property below the normal minimum of 0.50% or 0.60% may be considered where flatter • environmental considerations grades allow the retention, rather than the removal, of existing pavements. • in urban areas, adjacent land use 4. The minimum gradients outlined are 7. Maximum grades of 3 to 5% are considered suitable for normal conditions of rainfall and appropriate for design speeds of 100 km/h drainage outlet spacing. Where less than and higher. This may have to be modified the normal minimum gradient is utilized, the in regions with severe topography such as lengths of such grades should be limited to mountainous terrain, deep river valleys, and short distances, and their location and large rock outcrops. frequency become important considerations. In special cases, hydraulic 8. Maximum grades of 7 to 12% are analysis is required to determine the extent appropriate for design speeds of 50 km/h of water spread on the adjacent travel lane. and lower. If only the more important False grading, (where the pavement grade roadways are considered, 7% or 8% would is not parallel to the top of curb), to ensure be a representative maximum grade for a adequate drainage is an effective means design speed of 50 km/h. of maintaining minimum grades in flat, highly constrained areas. False grading is 9. Control grades for other speeds between addressed in Section 2.1.5, Cross-Slope. 50 km/h and 100 km/h are intermediate between the above extremes.

September 1999 Page 2.1.3.3 Alignment and Lane Configuration

5. At intersections, minimum gradients are One of the properties of the parabola is that the particularly important to avoid operational rate of change of grade with respect to length safety concerns related to ponding or icing is constant. For this reason sight distance conditions. available to a driver travelling on a crest curve is constant throughout the length of the curve. 6. Ensuring positive drainage is a key element This is one of the reasons for the use of the in the grading designs of curb returns and parabola for vertical curves. A second the large paved surfaces of intersection advantage of the parabolic curve is that its areas. Suggested guidelines include a calculation is much simpler than a circular or minimum gradient of 0.6% along curb other curve that might be considered. returns, and a minimum of 1.0% combined crossfall and longitudinal gradient within the Since the rate of change of grade is constant limits of an intersection. Further information with respect to length, this property is used to on intersection drainage considerations is designate the size of the curve. The length of a contained in Chapter 2.3, Intersections. section of curve measured horizontally over which there is a change of grade of 1% is a 7. For gravelled roadways or public lanes, a constant for the curve and is referred to as the longitudinal grade of 0.8% or more is K value. For example, a K value of 90 means a desirable to ensure adequate surface horizontal distance of 90 m is required for every drainage, unless parallel ditches are one percent gradient change. K is a measure provided. A grade of 0.5% may be used of the flatness of a curve, the larger the K value as an absolute minimum. the flatter the curve, in the same way that radius is a measure of the flatness of a circular curve. Drainage around Curves For crest curves K is negative, and for sag curves K is positive. 1. Avoid areas of level grades around curves if there is possibility of raised medians being K = L / A (2.1.23) installed in the future. The installation of such a median as part of a road rehabilitation or widening project may preclude the possibility of providing Where: drainage in the median. Superelevated sections of pavement that drain towards a L = Horizontal length of vertical depressed median, and utilize longitudinal curve (m) grade in the median, may require longitudinal grade on the pavement for A = Algebraic difference of grade drainage, if the median is subsequently lines (%) raised. K = Parameter which has a 2.1.3.3 Vertical Curves constant value in a given vertical curve. Introduction In certain situations, because of critical The function of a vertical curve is to provide a clearance or other controls, the use of smooth transition between adjacent grades. unsymmetrical vertical curves may be required. Because their use is infrequent, the derivation The form of curve used for vertical curves is a and use of the appropriate formulae have not skewed parabola, positioned so that basic been included in the following section. For use measurements can be made horizontally and in such limited instances, refer to unsymmetrical vertically. Curves are described as crest or sag curve data found in a number of highway depending on their orientation. engineering texts.

Page 2.1.3.4 December 2009 Geometric Design Guide for Canadian Roads

Table 2.1.3.4 K Factors to Provide Minimum Stopping Sight Distance on Sag Vertical Curves1

Design Assumed Stopping Rate of Sag Vertical Curvature (K) Speed Operating Sight Headlight Control Comfort Control Speed Distance Calculated Rounded Calculated Rounded (km/h) (km/h) (m) 30 30 29.6 3.9 4 2.3 2 40 40 44.4 7.1 7 4.1 4 50 47-50 57.4-62.8 10.2-11.5 11-12 5.6-6.3 5-6 60 55-60 74.3-84.6 14.5-17.1 15-18 7.7-9.1 8-9 70 63-70 99.1-110.8 19.6-24.1 20-25 10.0-12.4 10-12 80 70-80 112.8-139.4 24.6-31.9 25-32 12.4-16.2 12-16 90 77-90 131.2-168.7 29.6-40.1 30-40 15.0-20.5 15-20 100 85-100 157.0-205.0 36.7-50.1 37-50 18.3-25.3 18-25 110 91-110 179.5-246.4 43.0-61.7 43-62 21.0-30.6 21-30 120 98-120 202.9-285.6 49.5-72.7 50-73 24.3-36.4 24-36 130 105-130 227.9-327.9 56.7-85.0 57-85 27.9-42.8 28-43

Values for sag curvature based on the comfort character of the terrain, is preferable to an criterion are shown in Table 2.1.3.4. alignment with numerous breaks and short lengths of grade. On lower speed curbed These K values for sag curves are useful in urban roadways drainage design often urban situations such as underpasses where it controls the grade design. is often necessary for property and access reasons to depart from original ground 2. Vertical curves applied to small changes of elevations for as short a distance as possible. gradient require K values significantly Minimum values are normally exceeded where greater than the minimum as shown in feasible, in consideration of possible power Tables 2.1.3.2 and 2.1.3.4. The minimum failures and other malfunctions to the street length in metres should desirably not be lighting systems. Designing sag vertical curves less than the design speed in kilometres along curved roadways for decision sight per hour. For example, if the design speed distance is normally not feasible due to the is 100 km/h, the vertical curve length is at inherent flat grades and resultant surface least 100 m. drainage problems. 3. Vertical alignment, having a series of 2.1.3.4 Vertical Alignment: successive relatively sharp crest and sag Design Domain curves creating a “roller coaster” or “hidden Additional Application dip” type of profile is not recommended. Heuristics Hidden dips can be a safety concern, particularly at night. Such profiles generally Vertical Alignment Principles: Application occur on relatively straight horizontal Heuristics alignment where the roadway profile closely follows a rolling natural ground line. Such The following principles generally apply to both roadways are unpleasant aesthetically and rural and urban roads. A differentiation between more difficult to drive. This type of profile is rural and urban is made in several instances avoided by the use of horizontal curves or where necessary for clarity. by more gradual grades.

1. On rural and high speed urban roads a 4. A broken back grade line (two vertical smooth grade line with gradual changes, curves in the same direction separated by consistent with the class of road and the a short section of tangent grade) is not

September 1999 Page 2.1.3.9 Alignment and Lane Configuration

desirable, particularly in sags where a full 10. On long grades it may be preferable to view of the profile is possible. This effect is place steepest grade at the bottom and very noticeable on divided roadways with decrease the grades near the top of the open median sections. ascent or to break the sustained grade by short intervals of flatter grade instead of a 5. Curves of different K values adjacent to uniform sustained grade that might be only each other (either in the same direction or slightly below the allowable maximum. This opposite directions) with no tangent is particularly applicable to low design between them are acceptable provided the speed roads and streets2. required sight distances are met. 11. To ensure a smooth grade line on high 6. An at-grade intersection occurring on a speed routes a minimum spacing of 300 m roadway with moderate to steep grades, between vertical points of intersection is should desirably have reduced gradient desirable. through the intersection, desirably less than 3%. Such a profile change is beneficial for 12. The design of vertical alignment should not vehicles making turns and stops, and be carried out in isolation but should have serves to reduce potential hazards. a proper relationship with the horizontal alignment. This is discussed in 7. In sections with curbs the minimum Section 2.1.4. longitudinal grade is 0.5%. Within superelevated transition areas, it might Drainage: Application Heuristics sometimes be virtually impossible to provide this minimum grade. In such cases, 1. Where uncurbed sections are used and the longitudinal grade length below 0.5% drainage is effected by side ditches, there should be kept as short as possible. is no limiting minimum value for gradient Additional information on minimum grades or limiting upper value for vertical curves. and drainage is provided in Subsection 2.1.3.2. 2. On curbed sections where storm water drains longitudinally in gutters and is 8. A superelevation transition occurring on a collected by catch basins, vertical alignment vertical curve requires special attention in is affected by drainage requirements. order to ensure that the required minimum Minimum gradients are discussed in curvature is maintained across the entire Subsection 2.1.3.2. width of pavement. The lane edge profile on the opposite side of the roadway from 3. The profile of existing or planned the control line may have sharper curvature stormwater piping is an important due to the change in superelevation rate consideration in setting urban roadway required by the superelevation transition. It grades. Storm sewer pipes typically have is, therefore, necessary to check both edge minimum depths to prevent freezing. These profiles and to adjust the desired minimum requirements are considered in setting vertical curvature. catchbasin elevations.

9. Undulating grade lines, with substantial 4. Where the storm sewer system is not lengths of down grade, require careful sufficiently deep to drain the streets by review of operations. Such profiles permit gravity flow, lift stations are an alternative. heavy trucks to operate at higher overall However, lift stations are generally speeds than is possible when an upgrade considered undesirable due to the high is not preceded by a down grade. However, costs associated with installation, operation this could encourage excessive speed of and maintenance. Malfunctions at the lift trucks with attendant conflicts with other station during a rain storm can also have a traffic. major detrimental impact on the street system and the adjacent developments.

Page 2.1.3.10 December 2009 Geometric Design Guide for Canadian Roads

Railways 2. It is desirable to allow up to 3.6 m of vertical clearance in order to provide an enhanced 1. Minimum vertical clearance over railways design and permit access for typical service is 6.858 m (22.5 feet) measured from base vehicles. of rail. 3. Similar vertical clearances are normally 2. The minimum vertical clearance where provided for sidewalks since cyclists may ballast lifts are contemplated is 7.163 m occasionally use the sidewalk, even where (23.5 feet) measured from base of rail not legally permitted. elevation to the underside of the overpass structure. 4. If it can be clearly determined that cyclists will not use the pedestrian sidewalk, 3. In all cases, it is good practice to confirm minimum clearances in accordance with the specific clearance requirements with the National or Provincial Building Codes the pertinent railway company, as well as could be employed. with the appropriate Federal and Provincial agencies, before designs are finalized. 5. Further information on sidewalks and bikeways is provided in Chapters 2.2, 3.3 Overhead Utilities and 3.4.

1. The vertical clearance requirements for Waterways roadways crossing beneath overhead utilities vary with the different agencies. In 1. Over non-navigable waterways, bridges the case of overhead power lines, the and open footing culverts, the vertical clearance varies with the voltage of the clearance between the lowest point of the conductors. The clearance requirements in soffit and the design high water level shall each case should be confirmed with the be sufficient to prevent damage to the controlling agency. structure by the action of flow water, ice flows, ice jams or debris. Pedestrian Overpasses 2. For navigable waterways, navigational 1. Normally, the minimum vertical clearance clearance is dependent on the type of for a pedestrian overpass structure is set vessel using the waterway and should be at 5.3 m or 0.3 m greater than the determined individually. clearance of any existing vehicular overpass structure along that same route. 3. Clearances should also conform to the This lessens the chances of it being struck requirements of the Navigable Waters by a high load - an important consideration Protection Act of Canada. - since a pedestrian overpass, being a relatively light structure, is generally unable Airways to absorb severe impact and is more likely to collapse in such an event. The increased 1. Vertical clearance to airways is as indicated vertical clearance reduces the probability in Figure 2.1.3.3. of damage to the structure and improves the level of safety for pedestrians using the 2. Lighting poles should be contained within structure. the clearance envelope.

Bikeways and Sidewalks 3. The dimensions are for preliminary design. Specific dimensions should be approved by 1. For bikeways, the minimum vertical the designated Transport Canada clearance provided is 2.5 m. representative.

December 2009 Page 2.1.3.13 Alignment and Lane Configuration

Figure 2.1.3.3 Airway Clearance1

2.1.3.5 Explicit Evaluation of collisions tend to get reported. Thus, the severity Safety and frequency of reporting collisions are affected by the grade. Second, road grades General affect the diversity of speeds. This is thought by some to affect collision frequency. Third, road Vertical alignment design has a significant profile affects the available sight distance and impact on safety in areas where vehicles are gradient affects braking distance. All of these required to frequently stop and start. Excessive factors may affect collision frequency and grades in intersection areas and at driveways severity. Finally, grade determines the rate at can contribute significantly to collision which water drains from the pavement surface frequencies during wet or icy conditions. Efforts and this too may affect safety. The traditional are normally made to provide as flat a grade as belief was that safety-related design attention practicable in these critical areas, while meeting should focus on crest curves and sag curves. the minimum slopes needed for adequate It turns out that while the vertical profile is an surface drainage. important determinant of the future safety of a road, sight distance at crest or sag curves is Collision Frequency on Vertical Alignment not as important as it seemed.

The following information on collision frequency At present the quantitative understanding of how is derived from a 1998 research paper29. grade affects safety is imprecise. All studies using data from divided roads concluded that The vertical profile of a road is likely to affect collision frequency increases with gradient on safety by various mechanisms. First, vehicles down grades. Some studies concluded that the tend to slow down going up the grade and speed same is true for up grades, while others up going down the grade. Speed is known to concluded to the contrary. Estimates of the joint affect collision severity. Thus on the up grade effect of grade on both directions of travel vary. collisions tend to be less severe than on the It is suggested that the conservative Collision down grade. Since down grade collisions tend Modification Factor of 1.08 be used for all roads. to be more severe, a larger proportion of That is, if the gradient of a road section is

Page 2.1.3.14 December 2009 Geometric Design Guide for Canadian Roads

2.1.4 COORDINATION Examples of good and poor application of the above principles are illustrated in AND AESTHETICS Figures 2.1.4.1 to 2.1.4.12. Each photograph or perspective sketch has a brief comment 2.1.4.1 Introduction describing the significant visual qualities.

The visual aspect of the road as viewed by the 2.1.4.2 Alignment Coordination: driver and passenger is considered to be an Technical Foundation important feature of geometric design. The provision of visual comfort helps make driving a The principal guides for horizontal and vertical more relaxing experience resulting in better and alignment are set out in Sections 2.1.2 and safer traffic operation. Features which are 2.1.3. A section of road might be designed to aesthetically disturbing to the motorist are to be meet these guides, yet the end result could be avoided. An unsightly road is a blight on the a facility exhibiting numerous unsatisfactory or landscape whereas an aesthetically pleasing displeasing characteristics. Horizontal and facility can become an asset, enhancing the area vertical alignments are permanent design through which it passes. elements for which thorough study is warranted. It is extremely difficult and costly to correct To produce an aesthetically pleasing facility the alignment deficiencies after the road is designer requires an appreciation of the constructed. On freeways there are numerous relationship between the road and its controls such as multilevel structures and costly surroundings. Some specific principles to be right of way. On most arterial streets heavy considered include: development takes place along the property lines, which makes it impractical to change the • blending of the road with the surrounding alignment in the future. Thus, compromises in topography alignment designs must be weighed carefully. Any initial savings may be more than offset by • developing independent alignments for each the economic loss to the public in the form of roadway of a divided facility when right of collisions and delays. way permits

• continuous curvilinear design rather than It is difficult to discuss the combination of long-tangent, short-curve design horizontal alignment and profile without reference to the broader subject of location. The • integration of horizontal and vertical subjects are mutually interrelated and what may alignment be said about one generally is applicable to the other. It is assumed here that the general • implementation of designs with visually location has been fixed and that the problem pleasing structures, retaining walls and remaining is the specific design and landscaping harmonizing of the vertical and horizontal lines, such that the finished road or street will be an In many cases the above principles can be economical, pleasant, and collision-free facility achieved at an acceptable extra cost. In cases on which to travel. The physical controls or where additional cost is a factor, the benefits are influences that act singularly or in combination assessed against expenditure. In considering the to determine the type of alignment are the costs and benefits of design trade-offs of this character of road justified by the traffic, sort, the designer should ensure that safety- topography, and subsurface conditions, existing related factors are considered explicitly. In cultural development, likely future addition, many of the basic elements of design developments, and location of the terminals. An coordination contribute to the design consistency initial design speed is established when aspects of road design, and should thus be determining the general location, but as design considered in that context as well. The issue of proceeds to more detailed alignment and profile design consistency is discussed in Chapter 1.4 it assumes greater importance, and the speed of this Guide. chosen for design acts to keep all elements of

December 2009 Page 2.1.4.1 Alignment and Lane Configuration design in balance. The final design speed chosen combination with horizontal curvature may may be different from the initial design speed. result in a series of humps visible to the Design speed determines limiting values for driver for some distance, an undesirable many elements such as curvature and sight condition as previously discussed. The use distance and influences many other elements of horizontal and vertical alignments in such as width, clearance, and maximum combination, however, may also result in gradient; all are discussed in the preceding parts certain undesirable arrangements, as of this chapter. discussed later in this section.

Horizontal and vertical alignments should not be 3. Sharp horizontal curvature should not be designed independently. They complement each introduced at or near the top of a other, and poorly designed combinations can pronounced crest vertical curve. This spoil the good points and aggravate the condition is undesirable in that the driver deficiencies of each. Horizontal and vertical cannot perceive the horizontal change in alignments are among the more important of alignment, especially at night when the the permanent design elements of the road. headlight beams go straight ahead into Excellence in their design and in the design of space. The difficulty of this arrangement is their combination increase usefulness and avoided if the horizontal curvature leads the safety, encourage uniform speed, and improve vertical curvature, i.e., the horizontal curve appearance. is made longer than the vertical curve. Suitable design can also be made by using During the location stage and the design phase design values well above the minimums for of a facility, the finished roadway is viewed in the design speed. three dimensions and the consequences of various combinations of horizontal and vertical 4. Somewhat allied to the above, sharp alignment on the utility, safety and appearance horizontal curvature should not be of the completed project are considered. introduced at or near the low point of a pronounced sag vertical curve. Because the 2.1.4.3 Alignment Coordination: road ahead is foreshortened, any significant Design Domain Application horizontal curvature assumes an Heuristics undesirable distorted appearance. Further, vehicular speeds, particularly of trucks, A number of application heuristics which can often are high at the bottom of grades, and assist the designer in preparing well- erratic operation may result, especially at coordinated and aesthetic plans are offered night. below. 5. On two-lane roads and streets the need for 1. Curvature and grades should be in proper safe passing sections at frequent intervals balance. Tangent alignment or flat curvature and for an appreciable percentage of the at the expense of steep or long grades and length of the road often supersedes the excessive curvature with flat grades are general desirability for combination of both poor design. A logical design that offers horizontal and vertical alignment. In these the most in safety, capacity, ease and cases it is necessary to work toward long uniformity of operation, and pleasing tangent sections to secure sufficient appearance within the practical limits of passing sight distance in design. terrain and area traversed is a compromise between the two extremes. 6. Horizontal and vertical alignments should be made as flat as feasible at intersections 2. Vertical curvature superimposed on where sight distance along both roads or horizontal curvature, or vice versa, streets is important and vehicles may have generally results in a more pleasing facility, to slow or stop. but it should be analyzed for effect on traffic. Successive changes in a profile not in

Page 2.1.4.2 September 1999 Geometric Design Guide for Canadian Roads

Figure 2.1.5.1 False Grading and Cross-Slopes

2.1.5.4 Cross-Slope Arrangements: away from the median. These alternates are Application Heuristics illustrated in Figure 2.1.5.2 (four lane divided, alternates A and B). The advantages of the The direction of the cross-slopes, or the cross- crown are storm water drains to both sides of slope arrangements for various classes of the roadway and it facilitates the treatment of roads, are described below and illustrated in the roadway with de-icing chemicals which are Figure 2.1.5.2. The rate of cross-slope in each spread in a narrow strip about the crown line, case depends on the type of surface as noted allowing the action of traffic and cross-slope to earlier, as well as, if relevant, on the width of further spread the chemicals across the entire roadways and drainage considerations. pavement. If the road eventually requires expansion to six lanes by adding two lanes in On tangent sections of roadway, cross-slope is the median, the additional lanes will slope normally applied to drain storm water to the side toward the median. The advantages of both of the roadway. On two-lane roads the lanes draining away from the median is the pavement is normally crowned at the centreline reduction in median drainage provision. and the pavement slopes down to each edge. If a four-lane divided road is to be expanded to On four-lane undivided roads and four-lane six lanes within a short period of time of initial divided roads with a flush median, the crown is construction, it is normally designed for six lanes normally placed in the centre of the pavement and built without the median lanes initially. In or median, and cross-slope to each pavement this case both lanes of each roadway slope edge is 0.02 m/m. toward the outer edge.

On a four-lane divided road with a depressed For six-lane divided roads, the crown for each median, a crown may be placed at the centre roadway is applied to either edge of the centre of each roadway with a cross-slope of lane, in which case one or two lanes drain 0.02 m/m to each edge, or both lanes may drain toward the median. With two lanes draining

September 1999 Page 2.1.5.3 Alignment and Lane Configuration

Figure 2.1.5.2 Application of Cross-slope on Various Types of Roads7

two-lane undivided

four-lane undivided

four-lane divided with narrow raised median

four-lane divided with wide depressed median (A)

four-lane divided with wide depressed median (B)

six-lane divided (A)

six-lane divided (B)

eight-lane divided

Page 2.1.5.4 December 2009 Geometric Design Guide for Canadian Roads toward the median, at locations where an For resurfacing, design guidelines and auxiliary lane is added, two lanes are draining acceptable limits are provided in Table 2.1.5.1 in each direction. This location of the crown is for pavement cross-slope related to design also convenient for an initial stage of four-lane speed. divided. If the six-lane cross section is to be a stage of an eight-lane cross section, a crown The cross-slope for the other types of lanes located at the common edge of the median and addressed later in the chapter, including centre lane is preferred to avoid three lanes climbing lanes, passing lanes and service roads draining toward the median. The above is generally adhere to the following criteria. illustrated in Figure 2.1.5.2 (six-lane divided, alternates A and B). For truck climbing lanes the cross-slope is usually handled the same as the addition of a Cross-slope on auxiliary lanes is the same as lane to a multilane road. Two common practices that of the adjacent through lane. are:

At intersections where two roads on tangent 1. The continuation of the cross-slope of the intersect, normal cross-slope is maintained on through lanes. the major road, and cross-slope on the minor road is run out on the approaches to the 2. A small increase in cross-slope compared intersection to match the profile of the major to the through lanes. On superelevated road. This treatment is typical of intersections sections the cross-slope is usually a controlled by a stop sign on the minor road. In continuation of the through lanes unless the case of an intersection where the two roads truck speeds are extremely slow and icy are of equal importance, or where the conditions prevail in which case an intersection is signalized, the normal cross- adjustment in the truck climbing lane cross- slope is run out on all four approaches so that slope may be desirable. the cross-slope on each road matches the profile of the crossing road. Simply put, the For passing lanes the practice is similar to that pavements are warped to maintain smooth for truck climbing lanes except that slow speeds profiles for traffic on both roads. This topic is are not an issue. dealt with in more detail in Chapter 2.3. The cross-slope for service roads, express For roadways on structures, the cross-slope is collector systems, and weaving lanes basically a minimum of 0.02 m/m. follow the same principles as would be applied to a similar class of through lane.

Table 2.1.5.1 Pavement Cross-Slope for Resurfacing1

Cross-Slope (m/m) Maximum Design Acceptable Algebraic Difference Design Speed Guidelines Limits (driving lanes) (m/m) 60 km/h 0.02 0.015-0.04 0.08 80 km/h 0.02 0.015-0.035 0.07 100 km/h 0.02 0.015-0.03 0.06 120 km/h 0.02 0.015-0.025 0.05

September 1999 Page 2.1.5.5 Alignment and Lane Configuration

2.1.5.5 Cross-Slope Changes: 100we Application Heuristics l = 2s (2.1.29) Sometimes it is necessary to change the cross- slope on tangents for reasons other than Where: w = the width of pavement (m) superelevation. As in superelevation it is important that the change is gradual enough to e = the change in super- provide visual driving comfort. This is achieved elevation developed (m/m) by having an acceptable relative slope which is a slope or profile of the outer edge of pavement s = the relative slope (%) in relation to the profile of the centreline. It is dependent on the rate of cross-slope being Table 2.1.5.3 Design Values for developed, the length over which it is developed, Rate of Change of and the width of the pavement. It is therefore Cross-Slope for an expression of rate of change of cross-slope. Single-Lane Turning The maximum relative slope normally applied Roads1 varies with design speed. Table 2.1.5.2 provides values for maximum relative slope for two-lane roadways. For four-lane and six-lane roadways, Design Speed Rate of Change (km/h) of Cross-Slope the lengths are increased by 1.5 and 2.0 times (m/m/m length) that for two-lane roadways, respectively. 25 and 30 0.0025 Design values for rates of change of cross-slope 40 0.0023 are shown on Table 2.1.5.3. These values are 50 0.0020 suitable for single-lane ramps. For two-lane 55 and more 0.0016 ramps lower values are appropriate. Theoretically, the maximum rate of change for two-lane roadways should be 50% of that for The phenomenon of adjacent traffic lanes single-lane roadways, however, this may having different rates of cross-slope or generate transition lengths which cannot be superelevation gives rise to a ridge at the achieved at an acceptable cost and values of common edge, referred to as algebraic 75% are acceptable. difference or roll-over.

The minimum transition length is given by the Too great a difference in cross-slope may cause equation: vehicles travelling between lanes to sway, giving

Table 2.1.5.2 Maximum Relative Slope Between Outer Edge of Pavement and Centreline of Two-Lane Roadway1

Design Speed (km/h) Relative Slope (%) 40 0.70 50 0.65 60 0.60 70 0.55 80 0.51 90 0.47 100 0.44 110 0.41 120 0.38 130 0.36

Page 2.1.5.6 December 2009 Geometric Design Guide for Canadian Roads

2.1.7 LANE AND ROUTE Illustration (iv) shows how the deficiencies of illustration (iii) can be resolved. The number of CONTINUITY AND lanes on each ramp and transfer roadway are WEAVING the same, proper lane balance is observed and all three basic lanes are continuous. This is 2.1.7.1 Lane Continuity accomplished by use of auxiliary lanes on both the left and right-hand sides of the roadway. Technical Foundation 2.1.7.2 Route Continuity1 In designing the lane arrangement of a freeway, design volume, maintenance of basic lanes and Technical Foundation lane balance are all taken into account. A further consideration is that of lane continuity. Route continuity refers to the provision of a directional path along and throughout the length A driver needs to recognize which lanes are of a designated route. Continuity of route basic or through, to avoid being inadvertently designation, either by name or number, is led by the lane markings to an undesired ramp important to ensure operational uniformity and lane. If good lane balance is applied and basic to reassure the driver that he is on the intended lanes are maintained, together with all exits and course. The principle of route continuity entrances having a single lane on the right, lane simplifies the driving task in that it reduces lane continuity will naturally follow. Where entering changes, simplifies signing, delineates the and exiting ramps have two or more lanes, and through route, and reduces the driver’s search at transfer lanes on collector roadways on for directional signing. express-collector systems, lane continuity can be lost. Desirably, the through driver, especially the unfamiliar driver, should be provided a Best Practices continuous through route on which it is not necessary to change lanes and through traffic Figure 2.1.7.1 illustrates examples of lane vehicular operation occurs on the left of all other continuity. Included is an example of non- traffic. In maintaining route continuity through compliance of lane continuity followed by an cities and bypasses, interchange configurations illustration of a rearrangement of lanes and need not always favour the heavy movement ramps in order to achieve lane continuity. In but rather the through route. To accomplish this, illustration (i) three basic lanes are maintained, heavy movements can be designed on flat all ramps are single lane on the right and the curves with reasonably direct connections and principles of lane balance are observed. Lane auxiliary lanes, equivalent operationally to continuity is maintained. In illustration (ii) there through movements. are three basic lanes, all ramps are on the right and have two lanes, lane balance is preserved Best Practices and lane continuity is maintained. Adherence to the above route continuity In illustration (iii) there are two single-lane principles influences the configuration of entrances on the right, a two-lane exit on the interchanges. In Figure 2.1.7.2, two continuity right, a two-lane (transfer) entrance on the left arrangements are illustrated. and a two-lane (transfer) exit on the left. Although three basic lanes are maintained In illustration (i) Highway 1 is a north-south route through the section and the principles of lane and Highway 2 is an east-west route, in which balance are observed, only one of the basic case a conventional four level fully-directional lanes is continuous. This confuses the driver, interchange is appropriate and the designated causes turbulence and unnecessary lane through routes are consistent with the route changing in the traffic operation, and is numbers. In illustration (ii), Highway A is an potentially hazardous. east-south route and Highway B is a north-west

September 1999 Page 2.1.7.1 Alignment and Lane Configuration

Figure 2.1.7.1 Examples of Lane Continuity1

Page 2.1.7.2 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.1.7.2 Illustration of Route Continuity1

December 2009 Page 2.1.7.3 Alignment and Lane Configuration route. The through routes, and the route names and the second is called a major weave with and numbers, are maintained and the ramps crown line. carry traffic between route numbers. Type B weave section is classified as a major If the conventional configuration of illustration weaving section because it involves multilane (i) were applied to the Highway A/B interchange, entry and/or exit lanes. Two critical the through route numbers would be carried on characteristics that distinguish Type B weaving ramps. This would confuse a driver who expects areas are: 1) one weaving movement may be to exit on a ramp (on the right) only when accomplished without making any lane departing from the through route number of changes, and 2) the other weaving movement another route. The designated through route requires at most one lane change. Type B name or number, therefore, influences the weaving sections are extremely efficient in selection of the configuration of the interchange. carrying large weaving volumes, primarily because of provisions of a through lane for one 2.1.7.3 Weaving of the weaving movements.

Technical Foundation Type C weaving sections are similar to Type B weaving sections in that one or more through Weaving sections are roadway segments where lanes are provided for one of the weaving the pattern of traffic entering and leaving at movements. The distinguishing differences contiguous points of access result in vehicle between Type B and Type C weave sections is paths crossing each other. Weaving sections the number of lane changes required for the may occur within an interchange, between other weaving movement. entrance ramps followed by exit ramps of successive interchanges, and on segments of Best Practices overlapping roadways. The weave section operations are an important consideration in the The conflict between entering and exiting traffic location of ramp terminals. tends to interrupt the operation of normal through traffic, precipitates turbulence in traffic If the frequency of lane changes in a weaving flow and has the effect of reducing service section is similar to that on an open road, the volumes and capacity. Undesirable weaving section is said to be “out of the realm of conditions may be alleviated by increasing the weaving”, but where they exceed the normal number of lanes in the weaving section or frequency the condition is described as increasing the length between successive “weaving”. entrance and exit gores. Weaving sections may be eliminated from the main facility by the There are three primary types of weaving selection of interchange forms that do not have sections which are determined by the weaving, or by incorporation of collector operational features such as number of entry distributor roadways. Although interchanges lanes, number of exit lanes, and their impact that do not involve weaving operate better than on how lane changing must take place. The those that do, interchanges with weaving areas three types of weaving sections are: Type A, nearly always are less costly than those without. Type B, and Type C. These three types of weaving sections are illustrated in If the above measures are not effective or not Figure 2.1.7.3. feasible for resolving weaving concerns, it may be necessary to eliminate certain turning Type A weaving section requires that each movements or relocate them elsewhere, in the weaving vehicle make one lane change in order interest of maintaining the operational integrity to execute the desirable movement. Type A of the freeway. Alternatively, a weaving condition weaving section is also broken into two distinct may be eliminated by separating the conflicting weaving types. The first is a one sided weave, traffic movements vertically by introducing a

Page 2.1.7.4 September 1999 Geometric Design Guide for Canadian Roads

Figure 2.1.7.3 Types of Weaving Sections8

September 1999 Page 2.1.7.5 Alignment and Lane Configuration grade separation of basket weave configuration. weaving lengths may be imposed by other These solutions are illustrated in Figure 2.1.7.4. constraints such as the location (spacing) of existing arterial roads. Such shorter weaving The minimum length, number of lanes, and lengths operate with varying levels of quality and capacity of the weaving sections are determined safety depending on local conditions and using procedures in the Highway Capacity features such as traffic volumes, sight distance, Manual Special Report 209, 3rd Edition, visibility, horizontal and vertical alignment, and Transportation Research Board, Washington, cross section elements. D.C. (Revised) 1997. Weaving sections longer than 1000 m will For efficient operation on freeways, weaving frequently be out of the realm of weaving. length between a freeway interchange and an arterial interchange normally should be in the Weaving length is measured from the point range of 800 m to 1000 m and between arterial where lane edges at the merge are 0.5 m apart interchanges in the range of 550 m and 700 m. to where lane edges at the diverge are 3.7 m, It is recognized that in many cases shorter illustrated in Figure 2.1.7.5.

Figure 2.1.7.4 Solutions for Undesirable Weaving1

Page 2.1.7.6 December 2009 Geometric Design Guide for Canadian Roads

In spite of these factors, the following general 6. Shoulders fulfil an important function as a conclusions can provide design assistance: refuge area for broken-down vehicles. Making adequate provision for the refuge 1. A number of studies indicate that increased of such vehicles is particularly important on shoulder width is more beneficial to safety high-volume facilities such as rural and at higher traffic volumes than lower urban freeways. In consideration of this volumes. fact, and of the consequent need to allow drivers and passengers room to move 2. There are some indications in the research around the vehicle for maintenance and/or that roads with wider shoulders may tend repair purposes, the design domain for to have collisions of greater severity. This shoulder width allows for widths up to 3.0 m. phenomenon may be due to the higher In instances where the lower limits of the running speeds that such wider shoulders design domain are used, particular attention may encourage in some instances. should be paid to providing a forgiving roadside. 3. Shoulders wider than about 2.0 m to 2.5 m increase the number of injury collisions in 7. The Collision Modification Factor (CMF) some circumstances. Once again, this related to shoulder width and annual phenomenon may be due to the higher average daily traffic (AADT) is shown on running speeds that wider shoulders may Figure 2.2.4.6. This Figure is based on run- encourage. It is thus logical that, in off-road and opposite direction collisions. situations where wider shoulders are to be If the information shown in Figure 2.2.4.6 used, particular attention should be paid to is to be applied to total collisions, an providing an appropriate forgiving roadside appropriate correction needs to be applied. design. For example, if half of the collisions on a given road are of the run-off-road and 4. The safety effect of wide shoulders on level opposite direction type, the CMF needs to and straight roads is less than on sharp be applied only to half of the total number horizontal curves and on roads with steep of collisions. If the “other” collision types grades. are affected by shoulder width, the CMF needs to be reduced further. 5. Wider shoulders tend to have fewer runoff-road and opposite direction collisions. 8. Provision of full shoulders instead of only However, they may in some instances be curb and gutter on multilane suburban associated with greater levels of ‘other’ highways is associated with a 10% lower types of collisions. collision rate.18

December 2009 Page 2.2.4.9 Cross Section Elements

Figure 2.2.4.6 Collision Modification Factor for Various Shoulder Widths versus Annual Average Daily Traffic 10

1.6

1.4

1.2

1.0 collision modification factor

0.8

0.6

Page 2.2.4.10 December 2009 Geometric Design Guide for Canadian Roads

2.2.5 MEDIANS AND possible. In any case, the median width should be in balance with the other elements of the OUTER cross section and the character of the area. SEPARATIONS An outer separation is that portion of an arterial 2.2.5.1 Technical Foundation street, road, expressway or freeway which physically separates the outside travel lanes of A median may be defined as that portion of a a roadway from an adjacent frontage/service road which physically separates the travel lanes road or collector road. The width of an outer of traffic in opposing directions. Median width separation is measured from the outer (right) is the lateral dimension measured between the edge of the travel lanes to the closest edge of inner (left) edges of the travel lanes and includes the parallel frontage/service road or collector the left shoulder, the gutter or offset widths, as road, and includes the shoulder and gutter or shown in Figures 2.2.1.1 and 2.2.1.2. offset widths, as shown in Figure 2.2.1.1.

A median is a safety device which provides A discussion regarding median barriers is some measure of freedom from interference presented in Chapter 3.1. of opposing traffic. Medians provide a recovery area for errant vehicles, storage area for 2.2.5.2 Freeway and Expressway emergencies, speed-change lanes for left-turn Medians: Application and U-turn traffic, and reduce headlight glare. Heuristics Medians add to a sense of open space and freedom, particularly in urban areas. 1. Rural freeways usually have depressed medians of sufficient width to allow the road Medians should be visible day and night and bed to drain into the median and to should be in definite contrast to adjacent travel eliminate the need for median barriers. lanes. Medians may be flush with, raised above, or depressed below adjacent travel lanes. 2. Median side slopes are kept flat so that a vehicle leaving the travelled lanes has an Median widths may be as narrow as 1.0 m and opportunity to recover control minimizing as wide as 30 m. Widths above 3.0 m are occupant injury and vehicle damage. usually associated with independent Overturning crashes are more frequent for alignments, in which case the roadways are deeply depressed medians with slopes of designed separately, and the area between is 4:1 or steeper for median widths of 6.0 to largely left in its natural state. Median widths 12.0 m.12 Slopes steeper than 4:1 should for rural typical sections are normally multiples be avoided and flatter slopes are desirable of 1.0 m and for urban typical sections multiples where feasible in terms of cost, drainage of 0.2 m. and property. See Chapter 3.1 for additional discussion of slopes. Medians may serve as escape routes and provide a clear zone for vehicles that are 3. Wide medians promote safety by reducing avoiding possible collisions with vehicles in their the possibility of collision by vehicles own lanes.11 The major uses of a median travelling in opposite directions; and they separation are to eliminate the risk of head-on promote a sense of well-being for the collisions and to control access. Increasing travelling public. Rural freeway medians median width reduces the frequency of cross- in the order of 20 m are common and may median collisions. Collision frequencies be as much as 30 m. Wider medians generally decrease with increasing median constitute separate alignment and widths; however, current research is insufficient independent design. to allow quantification of the collision rates versus median width.11 For that reason, 4. In metropolitan fringe areas it may be medians should be as wide as economically appropriate to build a rural freeway with a

December 2009 Page 2.2.5.1 Cross Section Elements

depressed median, recognizing that the however, to surface the median in a character of the area will become urban and contrasting texture and/or colour to alert the that future lanes will be required together errant driver travelling in the median. with a flush or raised median. The ultimate Widths of flush highway medians without cross section is designed and then median barriers can vary between 1.0 m elements removed to determine the and 4.0 m. depressed median width for the first stage. 2. Wider flush medians with barriers normally 5. In developing the ultimate cross section, the apply to high speed rural arterial roads. designer should be aware that the installation of median barriers increases the 3. Medians in urban areas may be either flush collision rate but reduces crash severity or raised. because of the reduction or elimination of head-on collisions.12 4. A median in an urban area normally does not have a barrier because it has to be 6. Consideration should be given to providing terminated at intersections and some sufficient median width to preclude median entrances, in which case the safety benefits barriers in the ultimate stage when the of the median are offset by the hazard of future lanes are added. the barrier ends. Where a barrier is applied, it is usually a concrete barrier in a flush 7. Medians for urban freeways normally are median. Shoulders are not normally either flush or raised with a median barrier. justified in urban areas, in which case the Median dimensions depend on shoulder concrete barrier is offset 0.5 m from the widths, barrier type, and the need for edge of travelled lane. Additional median provision of structure piers. See width might be required to accommodate Chapter 3.1 for a discussion of median illumination plant, bridge piers or traffic barriers. control devices.

8. The normal width of a left shoulder for an 5. Where provision for a left-turn lane is urban freeway ranges between 1.5 m and required, the median may require widening 2.5 m. Therefore the median width should to provide the appropriate lane width, be at least 3.0 m plus the width of the rounded up to a multiple of 0.2 m. selected barrier plus allowances for such factors as barrier deflection on impact if 6. Additional width may be required for bridge median barrier systems are used, piers regardless of whether barrier illumination poles, overhead sign footings protection is provided. Figure 2.2.5.2 gives and bridge piers. Further discussion on suitable dimensions for raised median barriers is contained in Chapter 3.1. treatment at bridge piers for arterial roads.

9. Typical freeway medians are shown in 7. Additional median width may be required if Figure 2.2.5.1. heavy volumes of multi-trailer trucks are anticipated at unsignalized intersections. 2.2.5.3 Arterial Road Medians: Application Heuristics 8. Medians on a divided urban street serve a variety of important purposes related to 1. A flush median without barrier may be safety, traffic operations, access control, appropriate for rural highways with low to and aesthetics including: medium volumes and operating speeds. This median is normally slightly crowned • physical separation of opposing traffic to effect drainage, and is normally paved, flows often in the same surface material as the • adjacent lanes. It is advantageous, storage area for left-turning vehicles out of the path of the through traffic stream

Page 2.2.5.2 September 1999 Geometric Design Guide for Canadian Roads

2.2.7 CURBS AND 3. In addition, the barrier type curb can contribute to a high speed errant vehicle GUTTERS vaulting over a semi-rigid traffic barrier under certain conditions. For this reason, 2.2.7.1 Technical Foundation barrier curb is generally not used on urban freeways and is considered undesirable on Curbs are raised or vertical elements, located expressways and arterials with design adjacent to a travelled lane, parking lane or speeds in excess of 70 km/h. Barrier curb shoulder. They may be employed with all types is never used in combination with rigid of urban streets for any or all of the following concrete barrier systems. reasons: 4. Semi-mountable curb is considered to be • drainage control mountable under emergency conditions. Its face slope ranges from 0.250 m/m to • delineation of the pavement edge or 0.625 m/m with a maximum vertical height pedestrian walkways to improve safety of 125 mm. Semi-mountable curbs are used on urban freeways, expressways, and • right-of-way reduction with the elimination on high speed arterials (design speed over of open ditch drainage 70 km/h) as a trade-off between drainage requirements and, when required, the • reduction in maintenance operations functional needs of semi-rigid traffic barrier systems. • access control or provision 5. Mountable curb contains a relatively flat • aesthetics sloping face (0.10 m/m to 0.25 m/m) to permit vehicles to cross over it easily. While Curbs are used only to a limited extent on rural mountable curb may be used in conjunction roadways where drainage is usually controlled with either semi-rigid or rigid traffic barrier by means of drainage channels. Concrete systems, it is preferable not to use a curb gutters are typically used to facilitate longitudinal in combination with either of these traffic drainage along urban roadways. They are often barrier systems. cast integrally with curbs, but may also have a “vee” shape when used adjacent to a concrete 6. The cross section dimensions of concrete traffic barrier. curbs and curbs with gutters vary between municipal jurisdictions. Standardization of 2.2.7.2 Curbs: Best Practices curb and gutter dimensions within a given jurisdiction is desirable for economy and There are three general types of curb: barrier, uniformity in construction and maintenance semi-mountable and mountable (see practice. Figure 2.2.7.1). Each type may be designed as a separate unit or integrated with a gutter to 7. When introducing a curb at the transition form a combination curb and gutter section. between typical rural and urban road cross sections, the curb on the urban section is 1. Barrier type curb is vertical or near vertical, normally flared out to match the edge of with a typical height of 150 mm, and is shoulder on the rural section. Flare rates intended primarily to control drainage and of 24:1 for a design speed of 80 km/h and access, as well as to inhibit low speed 15:1 for 50 km/h are considered vehicles from leaving the roadway. appropriate. The end of the curb is normally tapered down to be flush with the shoulder 2. When struck at high speeds, barrier curbs surface to prevent blunt impacts between can result in loss of vehicle control and - in the curb and vehicle tires or snow clearing spite of its name - is inadequate to prevent equipment. a vehicle from leaving the roadway.

December 2009 Page 2.2.7.1 Cross Section Elements

Figure 2.2.7.1 Curb and Gutter Types

Page 2.2.7.2 September 1999 Geometric Design Guide for Canadian Roads

preferably by using a quantitative benefit 6. Horizontal clearances to a rigid barrier on cost analysis technique and documented overpass bridges on arterial roads are by the road designer in the process of shown on Figure 2.2.10.3 while horizontal reaching such a decision. clearances to a rigid barrier on overpass bridges on freeways are provided on Figure 3. Guidelines for minimum horizontal 2.2.10.4. The rigid barrier assists in keeping clearances on bridges on urban local and errant vehicles on the bridge structure and collector roads are provided in is preferred where pedestrians are Table 2.2.10.1. accommodated on the overpass.

4. Guidelines for minimum horizontal 7. Appropriate barrier transitions should be clearance at bridges on rural roads are used on the approaches to the structure.1 shown in Table 2.2.10.2. Refer to Chapter 3.1 for additional information. 5. Guidelines for minimum horizontal clearances at bridges on urban arterial 2.2.10.4 Vertical Clearances: Design roads and urban freeways are shown on Domain and Application Figures 2.2.10.1 and 2.2.10.2. Heuristics

Refer to Chapter 2.1.

September 1999 Page 2.2.10.5 Cross Section Elements

Table 2.2.10.2 Horizontal Clearance at Bridges on Rural Roads

Design Short Overpass (<50 m) Long Overpass (>50 m) Left Right Left Righ t Sp eed (km/h) No No Sidewalk Sidewalk Sidewalk Sidewalk

Undivided 50 1.2 0.5 1.0 1.0 Local 60 1.2 0.5 1.0 1.0 70 1.2 0.5 1.0 1.0 80 1.2 0.5 1.2 1.0 90 1.2 0.5 1.2 100 1.2 0.5 1.4

Undivided 60 1.5 1.0 1.2 1.0 Collector 70 1.5 1.2 1.2 1.0 80 2.0 1.2 1.0 1.0 90 2.0 1.5 1.2 100 2.5 1.5 1.4

Divided 70 1.2 1.5 1.2 1.0 1.2 1.0 Collector 80 1.2 2.0 1.2 1.0 1.2 1.0 90 1.2 2.0 1.5 1.0 1.2 100 1.2 2.5 1.5 1.0 1.4

Undivided 80 2.5 1.5 1.5 Arterial 90 2.7 1.5 1.5 100 3.0 2.0 1.6 110 3.0 2.5 1.7 120 3.0 2.5 1.8 130 3.0 2.5 1.8

Divided 80 1.5 2.5 1.0 1.5 Arterial 90 1.5 2.7 1.0 1.5 100 2.0 3.0 1.0 1.6 110 2.0 3.0 1.0 1.7 120 2.0 3.0 1.0 1.8 130 2.0 3.0 1.0 1.8

Freeway 100 2.5 3.0 1.5 2.0 110 2.5 3.0 1.5 2.0 120 2.5 3.0 1.5 2.0 130 2.5 3.0 1.5 2.0

Notes: 1. For short overpasses (<50 m) shoulder widths should be carried across bridge. 2. All clearances should meet requirements for sight distance.

Page 2.2.10.6 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.2.11.2 Nomograph for Predicting Utility Pole Collision Rate14

average annual daily traffic (vehicles/day) utility pole collision frequency (collisions/km/year)

December 2009 Page 2.2.11.3 Cross Section Elements

Page 2.2.11.4 September 1999 Geometric Design Guide for Canadian Roads

Figure 2.2.12.2 Staging of a New Four-Lane Undivided Arterial Street

method A method B

stage I ROW ROW ROW ROW sidewalk shoulder drainage ditch temporary curb or shoulder

stage II L C curb curb ROW ROW ROW ROW (existing) (existing) (existing) sidewalk sidewalk

December 2009 Page 2.2.12.5 Cross Section Elements

Page 2.2.12.6 September 1999 Geometric Design Guide for Canadian Roads

Figure 2.3.1.2 Typical Traffic Movements Within an Intersection and its Approach

12345678901234123456789

1234567890123123456789 4

123456 12345678901234

123456

December 2009 Page 2.3.1.5 Intersections

Diverging and merging may be to the right, to designer should be cautioned, however, that the the left, mutual or multiple. number of conflict points for offset, or split T- intersection arrangements, as shown in Figures Crossings are termed “direct” if the angle of 2.3.1.1 and 2.3.2.1, would not be 18 (2 x 9); intersection is between 70° and 110° (right- instead the number of conflict points for this type angled intersection) or “oblique” if the intersection of intersection configuration (two T- angle is less than 70° or greater than 110° (oblique intersections) would likely be larger than at a intersection). single cross-intersection with 32 conflict points.

Weaving consists of the crossing of traffic The conflict areas are divided into two streams moving in the same direction. It is categories: accomplished by a merging manoeuvre followed by a diverging manoeuvre. Weaving • major conflict areas where head-on, right- sections may be considered to be simple or angle or rear-end collisions may occur multiple with a further subdivision into one-sided or two-sided weaving. • minor conflict areas where sideswipe collisions may take place Conflicts Illustrations of traffic conflict areas are shown Every rural and urban at-grade intersection has in Figure 2.3.1.4.1 It should be noted that the conflict areas. One of the main objectives of 90o T- and cross-intersections have the intersection design is to minimize the severity smallest conflict areas in comparison to the of potential conflicts between all intersection skewed cross-intersection and the multi-legged manoeuvres. intersection which have the largest.

A traffic conflict occurs whenever the paths Channelized intersections with auxiliary lanes followed by vehicles diverge, merge or cross. further reduce the conflict area size and the number of vehicles passing through the same The number of traffic conflicts at intersections intersection point by separating traffic depends on: movements into definite paths of travel using pavement markings and islands. For further • the number of one-way or two-way information on channelized intersections, see approaches to the intersection Section 2.3.6.

• the number of lanes at each approach In urban environments especially, conflicts can also occur between vehicles and pedestrians, • signal control and vehicles and bicyclists. Vehicles typically conflict with pedestrian crossing manoeuvres. • traffic volumes Vehicles can conflict with any bicycle manoeuvre. The 90° T- and cross-intersections • the percentage of right or left turns are the most straightforward intersections for pedestrian and bicycle manoeuvres, 1,6 As an example, Figure 2.3.1.3 shows conflict channelization may increase vehicle/pedestrian points for a T-intersection (three-legged) and a conflicts as pedestrians attempt to cross the cross-intersection (four-legged). With the turning roadway. addition of a single intersection leg, the number of conflict points increases from 9 to 32. The Prohibited Turns difference in collision rates at three- and four- legged intersections is also illustrated on Prohibited turns can be discouraged by designing Figure 2.3.1.3. It is shown that as traffic volume tight or extended curb returns which make it increases, the role the increased number of difficult to achieve these turns. Channelization conflict points at a four-legged intersection plays is also used to restrict or prevent prohibited, in collision rate, becomes more significant.6 The undesirable or wrong-way movements. In

Page 2.3.1.6 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.3.1.3 Conflict Points and Collision Rates of Three- and Four-Legged Intersections 1,6

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1234567890123456712345678 8

1234567890123456712345679 conflict points8 8

123456789012345671234567 3 diverge 8 8 123456789012345671234567 3 merge 8 8 1234567890123456712345678 3 cross 8 123456789012345678

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December 2009 Page 2.3.1.7 Intersections

Figure 2.3.1.4 Conflict Areas at Intersections 1

roundabout

Page 2.3.1.8 September 1999 Geometric Design Guide for Canadian Roads

2.3.2 ALIGNMENT Where traffic on a minor roadway is, and will likely always be, controlled by a stop sign at an 2.3.2.1 Design Speed intersection, the design speed of the minor roadway can be reduced through the intersection area. As a basic requirement, it is The following is a discussion of design speed important to provide sufficient sight distance for as it pertains to intersections. Chapter 1.2 the design vehicle to safely depart from the presents discussion on roadway design speed. stopped position and make the desired Rural manoeuvre through the intersection.

In a rural environment, the design speed of the If a design speed equal to or greater than the major roadway is used for the main intersection existing posted speed cannot be achieved approaches to determine taper lengths, through an intersection, changes to the posted deceleration and acceleration lengths, and other speed, the implementation of speed advisory geometric features specific to traffic on the signing or similar treatment should be major roadway. considered. Sound judgement is called for in selecting the design elements that meet the Design speed is typically not reduced at rural expectations of the driver. intersections where drivers are accustomed to long periods of uninterrupted travel. Inattentive 2.3.2.2 Horizontal Alignment drivers should be alerted to the fact that an intersection is ahead and should have enough Intersections are ideally located on tangent time to react accordingly by providing adequate sections. Location of intersections on curves is deceleration and acceleration lengths, etc. for not desirable due to decreased visibility, the design speed. increased conflict potential for vehicles crossing the major roadway, and complications with Urban roadway superelevation and pavement widening on curves. Intersections on curves are In general, it is desirable to maintain the design discussed further in Subsection 2.3.2.5. speed of a roadway as it passes through an intersection, particularly for a roadway where It is desirable that intersecting roads meet at, the traffic has or may have the right of way or nearly at, right angles. through the intersection. Examples of this ° situation are: The benefits of a 90 angle of an intersection are: • an intersection controlled by traffic signals • or which may be controlled by signals in reduced size of conflict area (see the future Figure 2.3.1.4) • • a major road crossing a minor road where improved driver visibility the minor roadway has a stop or yield • control, and the major roadway is not more favourable condition for drivers to controlled judge the relative position and relative speed of an approaching vehicle and to • an uncontrolled intersection decide when to enter or cross the major road For an urban roadway controlled by a yield sign • at an intersection, approach speeds in the order reduced length of time of a crossing of 25 km/h are common. A suitable design manoeuvre speed for such an approach roadway within the • zone of the intersection would be 35 km/h. general decrease in severity of collisions (collisions occurring at an impact angle of

September 1999 Page 2.3.2.1 Intersections

90° are generally less severe than those The practice of realigning roads intersecting at occurring at angles of greater than 90°)1 acute angles in the manner shown in Figure 2.3.2.1 (A and B) is beneficial. Ideally, While crossing at 90° is preferable in most the curves used to realign the roads would avoid cases, it is occasionally necessary and even a decrease in operating speed along the advantageous to skew the crossing (for realigned roadway. The practice of constructing example, to favour a heavier turning short radii horizontal curves on minor roadway movement). However, angles less than 70°and approaches to achieve right-angle intersections greater than 110° are typically not desirable. For may be acceptable but not necessarily desirable example, at a skewed T-intersection with an in the urban and rural settings. These curves angle less than 70° certain undesirable result in increased lane infringements because conditions exist because of the flat angle of motorists tend to drive flatter curves by entry. Vehicles which do stop are standing in a encroaching on a portion of the opposite lane. position that affords poor visibility for the driver Also, the traffic control devices at the to judge the speed and the distance of intersection may be obscured resulting in the approaching vehicles on the major roadway. need for the installation of advanced warning Also, for a skew right, vehicles leaving the major signing.11 roadway to enter the minor roadway with a right turn are encouraged to do so at high speeds It should be noted that although examples C and for a skew left, drivers tend to cut the corner and D on Figure 2.3.2.1 provide poor network at higher speeds, thereby travelling in the continuity, both examples may be acceptable opposing lane for a considerable distance and alternatives. If implemented, suitable physical creating a safety concern.1 barriers or other obstructions should be placed across the former right of way of the minor road. Particular consideration should be given to These visual obstructions are desirable to alert maintaining an angle of skew within 10° of right the driver on the minor road that the road is angle (i.e. between 80° and 100°), when any of realigned and is no longer on continuous tangent the following conditions exist: through the intersection. Assuming a four-lane undivided arterial road, the split • two minor roadways with design hour T-intersection arrangement, example C (offset- volume (DHV) greater than 200 v/h (on both right), introduces back-to-back left turns on the roadways) intersect major roadway, which are generally undesirable unless left-turn auxiliary lanes can be provided. • minor roadway with DHV greater This layout, however, has the advantage of than 200 v/h intersecting with a major road requiring the driver, wishing to cross the major road, to select a gap in only the traffic • two major roads intersect approaching from the left, and then make a conventional right turn followed by a left-hand • either of the intersecting roadways has merge manoeuvre to reach the left-turn auxiliary more than two basic lanes lane. However, if no left-turn lane is provided, vehicles travelling along the minor roadway may • sight distance is at a minimum hold up traffic while waiting for a gap to turn left. With example D (offset-left), the turns introduced • design speed on either intersecting on the major roadway by the minor roadway roadway for through traffic is greater than 1 crossing manoeuvres are right turns only, which 80 km/h minimize the impact on through traffic on the major roadway. However, the driver attempting In the case of existing roads that intersect to cross on the minor roadway is required to ° ° ° ° between 70 and 80 (or 110 and 100 ) with select coincidental gaps in the traffic streams no collision or performance concerns, a from both directions on the major roadway. ° ° realignment to 80 (or 100 ) may not be cost Moreover, the driver is required to make effective. right-hand merge manoeuvres on the major

Page 2.3.2.2 December 2009 Geometric Design Guide for Canadian Roads operating conditions. If such combinations are intersections. The superelevation values are encountered, the intersection is normally based on the lateral friction factors from relocated or improved through realignment of Chapter 2.1. Each speed has a constant friction one or both of the intersecting roads, to improve factor; the variables are radius and safety. If the combined horizontal and vertical superelevation. alignment of the intersection creates poor visibility of the intersection, advance warning of The following illustrates the use of upcoming intersections should be provided Figure 2.3.2.9: through appropriate signing. 1. An intersection is located on a curved 6 The bar charts shown on Figure 2.3.2.8 illustrate roadway with a 200 m radius curve and a the differences in the relative collision rates design speed of 70 km/h. A superelevation between tangent roads both with and without rate is required to allow proper operation intersections and curved roads with intersections of the major through roadway. and with intersections on grade. 2. Chapter 2.1 shows 0.059 m/m In all cases, the provision of appropriate sight superelevation required for emax = 0.06 m/m. distances is important to promote collision free This would be inappropriate for an operation. The combination of horizontal and intersection. Figure 2.3.2.9 shows that the vertical geometry at an intersection should normal rate of superelevation through the produce traffic lanes that are visible to the driver intersection can be safely reduced to a at all times and provide a clear definition of the minimum 0.023 m/m. Given this situation, desired path of any permitted turn or direction the occupants of the vehicles will feel some of travel. decrease in comfort due to increased centrifugal force. 2.3.2.5 Reduced Superelevation Through Intersections The principles of superelevation runoff discussed in Chapter 2.1 for open roadway The general controls and considerations that conditions apply generally to intersections on determine the maximum rates of superelevation curves. The controls for the rate of change of for through roadways discussed in Chapter 2.1 cross-slope are primarily those of comfort and also apply to roadway sections with appearance, and vary with the design speed. intersections. However, within an intersection, As the design speed is reduced the length over drivers anticipate and accept operation with which a change in superelevation can be made higher lateral friction than they do midblock, is also reduced. Design values for rates of especially in an urban environment. As such, change of cross-slope are shown in lower rates of superelevation can be used on Table 2.3.2.1. curves through intersections. Superelevation commensurate with curvature

Typical maximum superelevation rates (emax) and speed is seldom practical through used at intersections are: intersections and at the terminals of turning roadways. In some cases, the following • rural areas: emax = 0.04 m/m to 0.06 m/m solutions can be used: • • urban areas: emax = 0.02 m/m to 0.08 m/m the through curving roadway cross section is widened at the intersection (see

The application of emax is discussed in detail in Chapter 2.1) Chapter 2.1. • the normal cross-slope of the through Figure 2.3.2.9 illustrates a suggested pavement is retained through the interrelationship between speed, lateral friction intersection and superelevation for roadways through

December 2009 Page 2.3.2.13 Intersections

Figure 2.3.2.8 Effect of Geometry on Intersection Collision Rates6

Page 2.3.2.14 September 1999 Geometric Design Guide for Canadian Roads

Two-Centred Compound Circular Curve control in terms of orderly movement of vehicles. The two-centred compound curve is the preferred design for all types of large trucks and When pedestrians are a consideration at a usually fits the minimum inside sweep of a signalized wide open-throat intersection the design tractor trailer combination adequately. “walk” and clearance times may be affected, Although a three-centred curve better fits the hence provision of adequate service and inside sweep of a tractor trailer combination, a protection for pedestrians may be required.1 number of benefits to using a two-centred curve over a three-centred curve have been identified: Three-Centred Compound Circular Curve

• less pavement area for two-centred curves To fit the edge of pavement closely to the than for three-centred curves minimum inside sweep of a tractor trailer combination, the application of a symmetrical • intersecting road vehicles are forced to arrangement of three-centred curves has proceed slowly with two-centred curves proven advantageous. This design is the practical equivalent to a curve transition for most • stop sign can be placed closer to the or all of its length. A three-centred curve is intersecting road centreline (more visible) typically used at a major intersection with with two-centred curves exclusive left- or right-turn lanes. In an operational sense, it is superior to the minimum • two-centred curve design tends to be more circular arc design because it better fits the inner economical rear wheel turning path of a tractor trailer, while providing some margin for driver error and In addition, a two-centred curve may be used requiring less pavement. to lay out the right edge of pavement for vehicles making a right turn from the minor roadway and Three-centred curve design for angles of turn a three-centred curve could be used for the right more than 90o may result in unnecessarily large shoulder for vehicles making a right turn from paved intersections, portions of which are often the major roadway. unused. This situation may lead to confusion among drivers and present a hazard to 1 Figure 2.3.4.5 illustrates the application, symbol pedestrians. These conditions may be alleviated and nomenclature of the two-centred to a considerable extent by the use of compound circular curve elements. asymmetrical three-centred compound curves, or by using large radii, coupled with corner For large tractor trailer combinations the islands. In Figure 2.3.4.610 the elements of recommended radii combination should be three-centred symmetric/asymmetric checked with the appropriate template and compound curves are illustrated. adjusted if necessary. The clearance between the inner rear wheel and the edge of pavement The two-centred curve provides for tractor trailer should be 0.5 m preferably and not less than off-tracking, however, there is not as much room 0.25 m. When applying the template the vehicle for driver error as there is on the three-centred should be properly positioned within the traffic curve. lane at the beginning and end of the turn and the inner rear wheel path should clear the curve The use of a two-centred curve is permitted in with the indicated minimum clearance. The situations where a three-centred curve would application of the design vehicle template is normally be used, but cost to purchase extra described in Chapter 1.2. right of way is extreme or where surrounding roadway geometrics do not allow for the When facilitating large trucks in non-industrial application of a three-centred compound areas, consideration should be given to curve.10 channelization to avoid large paved areas that may be confusing to a driver and difficult to

December 2009 Page 2.3.4.7 Intersections

Figure 2.3.4.5 Edge of Pavement Design – Two-Centred Compound Curve1

Page 2.3.4.8 September 1999 Geometric Design Guide for Canadian Roads

Figure 2.3.4.6 Edge of Pavement Design - Three-Centred Compound Curve10

September 1999 Page 2.3.4.9 Intersections

2.3.4.4 Shoulders at Simple A uniform intersection shoulder width is Intersections designed where the intersecting roadways are of similar importance. An urban cross section with barrier curb does not typically include shoulders. However, where There are three types of shoulder treatments shoulders are provided, they are paved and at simple rural intersections: mountable curb and gutter is used. • gravel shoulders (rural) The rural roadway at intersections includes shoulders or equivalent lateral clearance • paved shoulders (rural) outside the edges of pavement. A shoulder at intersections is provided for the same reasons • concrete curb and gutter (rural and urban) as that for the open roadways. It is an area adjacent to the driving lane where a driver can Each type of shoulder treatment is applied at make a stop in case of an emergency. It can intersections with or without tapers or also provide width for the occasional oversized deceleration lanes. vehicle and may be used as a bypass lane for emergency vehicles. Edge of pavement delineators may be used in conjunction with either gravel or paved shoulder Due to improper vehicle operations, shoulders treatment at intersections. Generally, the at intersections are subject to deterioration at a application of delineators is discouraged as they faster rate than along open roadways. Edge of are often damaged or destroyed by turning pavement drop-off and gravel strewn onto the vehicles and their effectiveness is greatly pavement are main concerns which require reduced. They also cause maintenance frequent inspection and maintenance. This problems during snow removal operations. section deals with shoulder treatment at However, delineators may provide a guidance intersections designed to minimize these to the drivers exiting and entering the roadway concerns. at locations with restricted visibility and during poor weather conditions. At intersections the required shoulder width varies from a minimum of 0.5 m to that equal Shoulder treatment at intersections should be to the open roadway shoulder width. Where two evaluated and designed for each location based roads of different operational characteristics and on existing and anticipated future traffic volumes functions intersect, the shoulder width at the and operational considerations. intersection normally varies and serves as a transition from a wide shoulder on the main Gravel Shoulders roadway to a narrow shoulder on the minor roadways. Shoulder treatment at intersections is usually achieved by surfacing the shoulder with gravel, 1 Where the major roadway is designed with see Figure 2.3.4.8 . However, unstabilized auxiliary lanes, the shoulder width from the shoulders generally undergo consolidation with major roadway to the minor roadway is time and the elevation of the shoulder at the transitioned along the arc length of the edge of pavement edge tends to become somewhat pavement curve. lower resulting in pavement drop-off. Also turning manoeuvres contribute to gravel For the far side of the major roadway tracking onto the pavement area. As such, intersection, the shoulder width of the minor gravel shoulders in intersection areas require roadway is extended along the arc length of the regular maintenance. edge of pavement and transitioned to the shoulder width of the main roadway within the Paved Shoulders 30 m recovery taper length, as shown in Figure 2.3.4.7.1 At intersections, drivers of large trucks sometimes cut across the shoulder when

Page 2.3.4.10 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.3.5.1 Typical Right-Turn Taper Lane Design at T-Intersections1

Figure 2.3.5.2 Typical Right-Turn Taper Lane Design at Cross-Intersections1

September 1999 Page 2.3.5.3 Intersections

Table 2.3.5.1 Right-Turn Tapers Without Auxiliary Lanes

Design Speed (km/h) Taper Ratio Taper Length for Horizontal Curve(a), (R) (through roadway) w = 3.5 m (m) (m) 50 15:1 53 500 60 18:1 63 750 70 21:1 74 1000 80 24:1 84 1200 Note: a) Flat radii as indicated can be used rather than tangent alignments, for right-turn tapers.

2.3.5.4 Design Elements for Where: S = storage length (m) Right-Turn Tapers with Auxiliary Lanes N =design volume of turning vehicles (v/h) The length of an auxiliary lane is based on deceleration and storage requirements. L = length (m) occupied by each vehicle (see Chapter 1.2) Deceleration should occur exclusively within the auxiliary lane, although in an urban environment At signalized intersections, the storage lane deceleration (up to 15 km/h) over the bay taper length should accommodate about 1.5 times is normally tolerable (especially in a peak-hour the average number of vehicles to be stored condition). per cycle for roadways with design speeds of 60 km/h or less, and about twice the average Suggested taper and parallel lengths are shown number of vehicles for design speeds greater on Table 2.3.5.2 and illustrated on than 60 km/h. Figure 2.3.5.3. Adjustments for intersections on curves are discussed in Subsection 2.3.8.8. The storage length calculated above should be checked against capacity analysis to ensure 22 Auxiliary lanes can be developed using reverse an acceptable level of service. curves or straight line tapers; reverse curves are typically used in an urban environment with The required storage for two-lane operation is curb and gutter. one half that for a single-lane operation.

On high-speed roads the taper length to the Where there is a possibility that an auxiliary auxiliary lane should generally conform to that lane may be used for either storage or discussed in Section 2.4.6 - Interchange Ramps. deceleration, the length is determined for both conditions and the total is used in design. For Where auxiliary lanes are used for the storage urban and suburban roads, the left-turn lane of turning vehicles at unsignalized intersections, length tends to be used mainly for storage the length of the lane in addition to deceleration during peak hours (typically slower peak hour length and exclusive of taper, is usually based speeds require less length for deceleration) on the number of vehicles that are likely to and mainly for speed change at off-peak hours accumulate in two minutes (2 min). The storage (the queue length tends to be smaller but the length required is calculated by the following speed in off-peak tends to be greater). For formula and can be used for right- or left-turning auxiliary lane widths, refer to Chapter 2.2. vehicles: NL S = (2.3.3) 30

Page 2.3.5.4 December 2009 Geometric Design Guide for Canadian Roads

2.3.6.3 Guidelines for the design is more common in urban areas and is Application of typically applied in consideration of capacity and Channelization storage requirements.

Channelization may also be implemented for At the intersection of two local roads or a local any of the functions described in and a collector road, particularly in residential Subsection 2.3.6.1. areas, the right-turn design for a yield condition could be a simple radius without an island. Guidance in determining the need for channelization is also provided in other Merge Design publications.22 Merge right-turn designs are applicable for 2.3.6.4 Right-Turn Designs conditions where a turning speed of greater than 40 km/h is desired at the intersection for The right-turn channelization volume warrant capacity or operational reasons. Designs of this presently in use is approximately 60 v/h. Most type are normally used at freeway and of the research in this area has been limited expressway ramp terminals, and for and consequently difficulties in developing connections onto high-speed arterials. The applicable criteria exist.1 application of the merge design is also a function of volume. If the merging volumes are Figure 2.3.6.1 illustrates the typical layout and too high, the result can be congestion, in which dimensions for four types of right-turn designs: case an added lane design is preferred. stop, yield, merge and added lane. The form of traffic control should be selected to suit the The turning roadway (see Section 2.3.7) is design and to minimize conflicts between right- introduced by a right-turn auxiliary lane and/or turning vehicles, and left-turning and through a tapered approach, which provides the vehicles. necessary deceleration characteristics.

Stop Design Added Lane (Lane Away) Design

The right-turn design for a stop condition at an The added lane (lane away) right-turn design intersection normally consists of a simple radius is normally for high volume right-turning and does not require channelization. movements. This design is also appropriate at an intersection where an auxiliary lane is Information on right-turn designs with simple introduced for access purposes, as discussed radii is provided in Section 2.3.4 and 2.3.5 of in Chapter 3.2. this Guide. If the added lane is an auxiliary lane used for Yield Design access purposes, radii providing lower turning speeds, 40 km/h or less, are suitable. Where At major intersections, such as arterial/arterial the added lane is an additional through lane on or collector/arterial intersections, or within a high-speed road, the right-turn design can industrial areas, the right-turn design for a yield vary substantially. Where no right of way, condition is typically a three-centred curve with physical constraints or intersection spacing sufficient radii to provide a small island. In limitations are present and there are no industrial areas with minimal pedestrian activity, pedestrian crossing considerations, the right the raised island is normally omitted to provide turn is often designed to minimize the speed more manoeuvring area for large turning trucks. differential between the vehicles on the adjacent The three-centred curve may be preceded on through lane and the turning vehicles on the the approach leg by either a right-turn auxiliary added lane at the convergence point. lane or a taper. Turning roadways are addressed in Section 2.3.7. The auxiliary lane

December 2009 Page 2.3.6.3 Intersections

Figure 2.3.6.1 Typical Right-Turn Designs

Page 2.3.6.4 September 1999 Geometric Design Guide for Canadian Roads

2.3.6.5 Traffic Islands guide posts, a mounded earth treatment or appropriate landscaping. General Temporary Island Installations An island is a defined area between traffic lanes This type is usually constructed of asphalt for control of vehicle movements in intersection 1 areas or for pedestrian refuge. Islands may be curbing, precast bumper curbing or sand bags . raised areas or may be painted. In rural areas Such islands would typically be used in the two most desirable and commonly used construction work zones. treatments are the raised island with mountable Islands are grouped into three functional curbs and the painted island. In urban areas classes which are illustrated on Figures 2.3.6.21, barrier curbs are used to protect pedestrians 2.3.6.31 and 2.3.6.41 and are described below: and to reduce the risk of vehicles striking poles, etc. Directional Delineation and approach end treatment is Directional islands control and direct traffic critical to good channelization design. Island movements. They guide the driver into the delineation can be divided into the following proper channel for the intended route. types: Directional islands are of many shapes and sizes, depending upon conditions and Curbed Islands dimensions. A common form is one of triangular This type can be applied universally and shape to separate right-turning traffic from provides the most positive traffic delineation. through traffic. Mountable curbs should be used in most cases. Divisional In rural areas where curbs are not common, this treatment is often limited to islands of small Divisional islands, also called raised median to intermediate size. Pedestrian refuge islands islands, are introduced at intersections, usually are usually protected with barrier curb. on approach legs, to separate streams of traffic travelling in the same or opposite direction. Painted Islands These islands are particularly advantageous in This type of island is generally designed in controlling left turns at skewed intersections and urban or suburban areas where speeds are low at locations where separate channels are and space is limited. Application of this type of provided for right-turning traffic. island may be considered in rural areas in Two types of divisional islands are commonly advance of raised median island, where used: maintenance and snow removal make curbs undesirable, and where high approach speeds • opposing divisional islands (for T- (urban or rural) make a curb a potential hazard. intersections) However, snow accumulation can obliterate pavement markings. • offset divisional islands (for cross- intersections) Non-Paved Areas Formed by Pavement

Edges 1 These islands are shown on Figures 2.3.6.5 and 2.3.6.6.1 This type of island is usually used for larger islands at rural intersections where there is Where the roadway is on a tangent, reverse curve sufficient space and/or where added expense alignment is necessary to introduce dividing of curbs may not be warranted or may pose a islands. In rural areas where speeds are high, traffic hazard. This island type may be reversals in alignment should have radii of at supplemented by delineators on posts, other least 2000 m. A median on an approach leg may

December 2009 Page 2.3.6.5 Intersections

Figure 2.3.6.2 Directional Islands1

Page 2.3.6.6 September 1999 Geometric Design Guide for Canadian Roads

the parallel lane portion of the left-turn lane, to lane. The lane, median and gutter widths shown minimize the median length. are typical and vary in accordance with cross section requirements. Method “B” shows the centreline continuous through the intersection and the roadway The raised median, protecting the left-turn area, widened symmetrically. In this method, the is effective in clearly defining the through vehicle departure taper is continued beyond the paths and the left-turn storage area in all approach taper, enabling the nose of the weather conditions. Also, if accesses exist in introduced median to be on the left side of the close proximity to the intersection, the raised roadway centreline on the approach. The median reduces the type and number of turning- geometry results in a longer median length than vehicle conflicts within the zone of the that created by Methods “A” or “C”. intersection. However, in instances where the length available for the left-turn auxiliary lane Method “C” is similar to Method “B” in that the may not be sufficient to store all the left-turn roadway is widened symmetrically about the vehicles during peak periods, it is advantageous centreline. The departure taper commences to use a painted rather than a raised median near the beginning of the parallel lane portion area in advance of the left-turn lane. In this case, of the left-turn lane to reduce the median length. the painted median area can be used to provide The approach nose to the median is centred additional storage during occasional peak traffic on the roadway centreline. periods, reducing the problem of left-turning vehicles blocking the through lanes. Divided Roadway The approach and departure taper designs are Figure 2.3.8.8 illustrates a typical layout of a left- a function of the design speed of the roadway. turn lane and a right-turn lane along a divided For high-speed roads (design speeds roadway. The right-turn lane layout is also >70 km/h), the importance of using a gradual applicable to undivided roadways. taper cannot be over emphasized. Refer to Table 2.3.8.1 for approach and departure taper Left-Turn Slip-Around Treatment at geometry with design speed. T-Intersections

The characteristics of each of the three methods A left-turn slip-around can be introduced on a of introducing a median, as shown on two-lane roadway at T-intersections under the Figure 2.3.8.7, are described in the following following conditions: paragraphs. • where the left-turning volumes do not Method “A” illustrates the geometry for a median warrant a full left-turn lane but are sufficient introduced totally to the left of the roadway to potentially affect through traffic centreline. A lateral shift is not required for the traffic approaching the intersection. For this • where through vehicles bypassing condition to occur on both approaches to a occasional left-turning vehicles throw gravel single intersection, the centrelines of the from the shoulder onto the roadway approach roadways must be offset from each other. Although this is a desirable means of The slip-around design is comprised of an introducing a median, it is a rare case, occurring auxiliary lane and tapers at each end, as shown only where excess right of way is available, in Figure 2.3.8.91. See Subsection 2.3.8.3 for where the roadways are not centred within the taper lengths. right of way, or where the rights of way are offset appropriately across the intersection. In this Usually the slip-around design is not applied method, only the lanes leaving the intersection on four-lane undivided roadways; however, are required to taper back to the normal where the left-turn lane is not warranted and undivided roadway cross section. The departure turning vehicles impede the through traffic, the taper typically commences at the beginning of slip-around has its merit.

December 2009 Page 2.3.8.11 Intersections

Figure 2.3.8.8 Turning Lane Design, Raised Median

Page 2.3.8.12 September 1999 Geometric Design Guide for Canadian Roads

Figure 2.3.8.15 Triple Left-Turns19

September 1999 Page 2.3.8.21 Intersections

e) Approach and Departure Lane Widths g) Roadway Delineation and Signage Considerations Left-turn approach lane widths used at a triple left-turn have been at least 3.3 m. Similarly, Even though intersection geometry may be downstream departure lane widths have been adequate to accommodate three-abreast left- designed to an absolute minimum of 3.5 m with turn movements, roadway delineation and a desirable width of 3.7 m. A key factor signage are equally important to the safe controlling the geometry of the downstream operation of the facility. Advance overhead receiving throat width is the tracking path of the signage of the triple left-turn lane configuration design vehicle as it transitions from a circular is a critical element to inform motorists of the to a tangential motion. The tracking path intersection and lane options. Turn lanes and approximates a spiral as the design vehicle paths should also be clearly delineated to avoid completes the left-turn movement. Therefore, driver confusion. the width of the clear portion of the intersection may need to be widened based on the design h) Summary vehicle turning characteristics. The turning geometry may be accommodated by setting the As with all geometric designs, site-specific median island nose on the receiving crossing conditions must be reviewed and good roadway a significant distance back from the engineering judgement applied to each design intersection. A 0.6 m offset from the vehicle of triple left-turn lanes. As more facilities are turning path in Lane 1 has been used in locating constructed and used by the motoring public, the median island nose. The receiving roadway additional research should be conducted to width at the intersection may also be widened include topics such as: by increasing the curb return radius of the opposite intersection quadrant. These • collision rate comparisons between dual geometric adjustments have to be carefully and triple left-turn lane installations evaluated for the intersection angle and roadway widths. Due to the high volumes of • lane utilization vehicle traffic, raised median islands of at least 0.6 m width (1.5 m desirable) should be used • determination of saturation flow rates for on the approach and departure legs of an triple left-turn lanes intersection with two-way traffic. Wider roadway median islands provide the intersection with • a comparison of left-turn capacity among larger radius curves thus improving the single, double and triple turn lanes intersection’s left-turning geometry. A raised median island has been found to provide a • the effects of downstream weaving on the driver in Lane 1 with a visual point of reference uniform loading of triple left-turn storage to guide a vehicle through the left-turn bays and intersection left-turn capacity19 manoeuvre. A raised median island also provides delineation for the stop bar location 2.3.8.7 Slot Left-Turn Lanes on the receiving roadway. This is especially important when the left-turn lane stop bar is General offset from the through movement to accommodate triple left-turn lane geometry. Slot left-turn lanes may be provided at intersections along major arterial roads or f) Auxiliary Lane and Taper Length expressways wherever a median of about 10.8 m or more in width is available. This width These are determined as per single and double is needed to accommodate a divisional island left-turn lanes. The total storage capacity of all between the left-turn lane and the adjacent lanes should be considered as well as through lane. Typical designs are shown on deceleration lengths. Figure 2.3.8.16. The major advantages are:

Page 2.3.8.22 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.3.8.18 Intersections on Curve

1234

December 2009 Page 2.3.8.27 Intersections

Figure 2.3.8.18 depicts shorter tapers with turning vehicles, especially those turning asymmetrical smoothing curves. The same against the superelevation. The superelevation criteria for adjusting taper lengths for curvilinear rate on an auxiliary lane at an intersection curve tapers can be applied to straight-line taper is normally the same as that of the through designs for turn lanes on the outside of main lanes. In restricted areas, it may be line curves. With the straight-line taper design, advantageous to reduce the superelevation however, shortening the taper on the inside of along the auxiliary lane (as compared to the the main line curve is not necessary. The start through lane) reflecting the lower running of the auxiliary lane on the inside of the curve speeds of the turning vehicles on the auxiliary remains distinctive due to the absence of the lane. smoothing curve. At intersections controlled with traffic signals, Alternatively, some jurisdictions lengthen reduced superelevation rates may be straight-line tapers along the inside of main line considered along the curved roadway to curves to provide the same rate of lateral shift improve the profile for the through vehicles on from the main line as that provided in a normal the cross roadway. For reduced superelevation tangent section. rates, as related to radius of curve and design speed, based on maximum lateral friction Superelevation factors see Chapter 2.1. It provides an alternate method of selecting superelevation rates based The maximum superelevation suggested on lower maximum lateral friction factors, which through an urban intersection area is 0.04 m/m. provides greater protection against skidding This allows reasonably smooth operation for during slippery pavement conditions.

Page 2.3.8.28 September 1999 Geometric Design Guide for Canadian Roads

2.3.9 TRANSITION diverging and merging values are shown in Table 2.3.9.11, as well as the design domain for BETWEEN parallel lane length ‘A’ beyond the intersection. FOUR-LANE ROADWAY AND Special consideration is given to the merging operation by providing increased taper lengths, TWO-LANE since it is recognized that merging is more ROADWAY AT critical when drivers, missing the warning signs, may be surprised by the sudden lane drop. INTERSECTIONS Length ‘A’ is needed for signing purposes.

2.3.9.1 Undivided Roadways 2.3.9.2 Divided Roadways

The lane arrangement for the transition from a Principles similar to those used for undivided four-lane to two-lane roadway, and conversely roadways are employed in the initial design from two-lane to four-lane roadway, is illustrated stages of a divided control access roadway, see 1 in Figure 2.3.9.1 . The typical taper lengths for Figure 2.3.9.21.

Figure 2.3.9.1 Transition Between Undivided Four-Lane Roadway and Two-Lane Roadway at an Intersection1

Table 2.3.9.1 Parallel Lane and Taper Lengths for Transition between Undivided Four-Lane Roadway and Two-Lane Roadway1

Length ‘A’ Merging Diverging Design Speed Design Domain Taper Taper (km/h) (m) (m) (m) 50 80 – 150 85 40 60 100 – 175 100 50 70 120 – 195 115 60 80 140 – 215 130 70 90 160 – 240 145 75 100 180 – 265 160 80 110 205 – 290 170 85 120 230 - 310 180 90

December 2009 Page 2.3.9.1 Intersections

Figure 2.3.9.2 Transition between Four-Lane Divided and Two-Lane Roadway Merge1

123

123 merging ‘B’

123

Page 2.3.9.2 December 2009 Geometric Design Guide for Canadian Roads

2.3.11 MEDIAN OPENINGS safely accommodate only U-turn movements may be desirable to improve access.

In certain circumstances, median openings or 2.3.11.1 Use and Function crossings may be provided to allow emergency vehicles to exit from adjacent fire, police and Openings in medians perform one of the ambulance facilities, or to make turns that are following functions: otherwise prohibited at intersections. On divided arterial roads with widely spaced intersections • accommodate cross traffic and left-turn and controlled access, median openings to movements at intersections permit only emergency vehicles to make U-turn movements may also be desirable. • permit left turns for access to adjacent development at locations other than 2.3.11.2 Elements of Design intersections (such development normally has significant traffic generation or other The design of a median opening and median special characteristics) ends should be based on traffic volumes, adjacent land use and the type of turning • permit U-turns on divided roadways at vehicles, as discussed in Chapter 1.2. Crossing locations other than intersections and turning traffic should operate in conjunction with the through traffic on the divided roadway. The use and function of median openings are As such, it is necessary to know the volume related to road classification. and composition of all movements occurring simultaneously during the design hours.12 The Median openings are generally not provided design of a median opening becomes a matter along freeways except for accommodation of of considering what traffic is to be U-turns for emergency and maintenance accommodated, choosing the design vehicle to vehicles. Along major divided arterials with fully use for layout controls for each through and controlled access, median openings are turning movement, investigating whether larger normally provided only at the intersections. For vehicles can turn without undue encroachment divided arterials where the provision of access on adjacent lanes, checking the intersection for to adjacent developments is only partially capacity, and evaluating the potential for controlled, median openings may be provided operational problems related to undesirable at entrances to major developments, such as driving behaviour. If the capacity is exceeded shopping centres, which generate significant by the traffic load, the design should be traffic volumes (refer to Chapter 3.2). The expanded, possibly by widening or otherwise provision of median openings may also be adjusting widths for certain movements. considered for other developments, such as shipping terminals, which generate significant The length of the median opening, measured volumes of large trucks. Such vehicles are nose to nose, is normally a function of the typically discouraged from making U-turn median width, the turning paths of the design movements within the road network and median vehicles making the left turn from and to the openings may be a preferred means of crossing roadway, and the location of the providing access where other alternatives are pedestrian crossings. If U-turns are permitted not possible. The spacing of the median at the median opening, additional length may openings normally conforms to the intersection be required to avoid conflicts. The bullet nose spacing guidelines provided in Section 2.3.1. design is advantageous in reducing the length of the median opening, thereby bringing the Where intersections along divided arterials are raised median end in close proximity to the widely spaced and access to adjacent pedestrian crossing and making it available for development is permitted, median openings that refuge.

December 2009 Page 2.3.11.1 Intersections

The minimum lengths of median openings This provision may be particularly important needed to accommodate varying turning radii where U-turns are selectively restricted or totally and median widths should be determined based prohibited by law at the signalized intersections. on the design vehicle (see Chapter 1.2). Median openings for U-turns reduce circulation on the adjacent local road system and improve The minimum length of the median opening, in access to adjacent developments by simplifying all cases, should be 12.0 m, or the width of the the manoeuvres necessary to reverse direction. crossing roadway measured between the outside edges of pavement plus 3.0 m, Normally, protected left-turn lanes are provided whichever is greater. in advance of the median opening for the U-turn manoeuvre. In areas with low traffic speeds and See Chapter 3.1 for discussion and further low U-turning volumes, the U-turn movement references on median barriers, end treatments may be permitted from the through lane. In and transitions. these limited cases, the median is normally of sufficient width to allow a single vehicle to stop 2.3.11.3 U-Turns in the median opening without encroaching into the paths of the through traffic. Median openings designed to accommodate vehicles making U-turns are needed on some Table 2.3.11.1 provides the minimum median divided roadways in addition to openings widths required for various design vehicles to provided for cross- and left-turning movements. make three types of U-turn manoeuvres on a The locations for separate U-turn median divided road: auxiliary lane to inner lane; openings are as follows: auxiliary lane to outer lane, four-lane divided roadway; auxiliary lane to outer lane, six-lane • beyond an intersection for accommodating divided roadway. The minimum median widths minor turning movements not otherwise for the latter two cases are based on through provided for in the intersection lane widths of 3.7 m. As demonstrated by the table, the median widths needed to • ahead of an intersection where through and accommodate the turning paths of trucks are other turning movements would be beyond the typical widths available in urban interfered with by U-turn movements at the areas. In special cases, where truck drivers intersection must be able to reverse their direction, other alternatives such as jug handle left turns may • at regularly spaced openings to be considered. accommodate maintenance and emergency vehicle operations 2.3.11.4 Emergency and Maintenance Vehicle Unless the median is wide, U-turning vehicles Crossings interfere with through traffic by encroaching on part of the through traffic lanes. U-turns are There are some locations where it is desirable made at low speeds and the required speed to allow only emergency and maintenance change normally is made on the through traffic vehicles to cross the median, including: lanes. Moreover, U-turns often require weaving to and from the other lanes of the divided • at fire, police or ambulance facilities along roadway. Allowing U-turns across narrow divided roadways medians where through traffic flow may be impeded is undesirable and may create safety • along divided roadways with fully controlled concerns. access and widely spaced intersections, and where median openings for all The provision of median openings specifically vehicular traffic are not desirable for U-turns is effective along divided roadways • where intersections are widely spaced and at intersections where it is desirable, from access to adjacent developments is permitted. a traffic operations perspective, to restrict

Page 2.3.11.2 December 2009 Geometric Design Guide for Canadian Roads

Table 2.3.11.1 Minimum Median Widths for U-Turns

Type of Manoeuvre (M) Minimum Width of Median for Design Vehicle (m) Passenger Single Tractor- Car Unit Truck Semitrailer Auxiliary lane to inner 12.1 22.1 24.5 lane

Auxiliary lane to outer 8.4 18.4 20.8 lane, four-lane divided

Auxiliary lane to outer 4.7 14.7 17.1 lane, six-lane divided

all but emergency vehicles from making median. The second treatment offers more through and left-turning movements positive control but often presents maintenance across the median and operational problems.

A variety of treatments can be implemented to The next two treatments are median crossings, limit the cross-median movements to rather than median openings, since the median emergency vehicles only. These include: remains visually closed. With a reduced median height of 30 to 50 mm, the median area usually • use of signing at a conventional median consists of a concrete slab suitable to opening to restrict all traffic except accommodate the types of emergency vehicles emergency vehicles likely to cross. Ripples or corrugations may be incorporated into the median surface, • use of automatic gates, in the median transverse to the direction of travel, to further opening, remotely controlled from within discourage the use of the median area by the adjacent emergency facilities non-emergency traffic.

• provision of a raised or rippled median with The last treatment provides a visible physical a reduced height of 30 to 50 mm, rather restriction to non-emergency traffic, but also than the normal raised median height of presents a significant obstacle to an emergency 150 mm vehicle making the crossing manoeuvre, since the median curb height is typically in the range • provision of semi-mountable or mountable of 75 to 125 mm. Crossing the median in this curbs at the crossing locations, and barrier case should be undertaken at low speed which curbs along the remainder of the adjacent may be a disadvantage. However, where it is median area highly desirable to discourage median crossings by other traffic, this treatment may be optimal. • installation of flexible or breakaway posts In this treatment the median crossing area, across crossing between the curbs, is paved with asphalt or concrete and the emergency vehicle turning The first treatment offers the least positive path is kept clear of signs, surface utilities or control or restriction to other vehicular traffic other appurtenances. and is typically appropriate where there is little need or desire for other traffic to cross the

September 1999 Page 2.3.11.3 Intersections

Page 2.3.11.4 September 1999 Geometric Design Guide for Canadian Roads

intersection), although complete reliance sensitive to increase in either, and considerable should not be placed on the roundabout scope exists for increasing capacity by various alone to act as an indicator to drivers combinations.

• emphasize the transition from a rural to an Increasing the entry radius R above 20 m only urban or suburban environment improves capacity very slightly. However, as values drop below 15 m capacity reduces at an • accommodate very sharp changes in route increasing rate. direction which could not be achieved by curves, even of undesirable radii The entry angle ‘∅’is fixed by the alignment of the approach roadways and there is, therefore, • provide a greater measure of safety at sites little scope for varying ‘∅’ sufficiently to have a with high rates of right-angle, head-on, left/ significant effect on capacity. through, and U-turn collisions When designing a roundabout the approach • replace existing all-way stop control width is a known fixed value. The capacity is thereafter almost totally determined by the entry • accommodate locations with low or medium width and the flare length, as typical values of traffic volumes, instead of signals the other geometric parameters have only a minor influence. Roundabouts should be sited on level ground preferably, or in sags rather than at or near the Reducing the inscribed circle diameter reduces crests of hills because it is difficult for drivers to capacity. If, however, by reducing the inscribed appreciate the layout when approaching on an circle radius an increase in the entry geometry up gradient. However, there is no evidence that can be achieved, then a large net increase in roundabouts on hill tops are intrinsically capacity is produced; mini roundabouts dangerous if correctly signed and where the (diameter less than 4 m) are the limiting case. visibility standards have been provided on the As the entry width increases, the entry deflection approach to the yield line. Roundabouts should is reduced and consideration should be given not normally be sited immediately at the bottom to safety. of long descents where the down grade is significant for large vehicles and loss of control Increasing the number of entry lanes or could occur. increasing the width of these lanes has the potential for increased traffic conflict. Widening Roundabouts may not be effective when the entry lanes is a concern for the safety of cyclists. flow of heavy vehicles is great or long delays on one approach exists. Table 2.3.12.1 Geometry/Capacity 2.3.12.4 Geometry/Road Capacity Relationships

As noted above, roundabouts can improve road Increase Capacity safety and increase capacity. Table 2.3.12.1 Parameter Change provides a summary of the relationship between the approach width V rises rapidly geometric parameters and capacity. the entry width E rises rapidly the flare length ll rises slowly Capacity is very sensitive to increases in the the entry angle ∅ drops slowly approach width V. This is normally the half width of the approach roadway and can only be the inscribed circle diameter rises slowly increased if sufficient roadway width allows the D centreline to be offset. the entry radius R rises slightly

The entry geometry is defined by the entry width E and the flare length ll. Capacity is extremely

September 1999 Page 2.3.12.3 Intersections

2.3.12.5 Safety Analysis United Kingdom for roundabouts and those developed in the United States for cross- Recent research in Europe has shown that intersections allow agencies to compare in collision rates can be decreased by replacing theory the safety of both types of intersections. conventional intersections with roundabouts. Both the U.K. and the U.S. models yield The Netherlands achieved a 95% reduction in estimates of collisions resulting in nonproperty 20 injuries to vehicle occupants at locations where damage. In addition, both models use state- roundabouts were installed.20 On inter-urban of-the-art regression analysis (Poisson and roads in France, the average number of negative binomial) and samples of sufficiently collisions resulting in injuries was 4 per large size to relate collisions to particular 100 million vehicles entering roundabouts, roadway characteristics. On the basis of these compared with 12 per 100 million vehicles similarities, one could draw the conclusion that entering intersections with stop or yield signs. roundabouts in the United States have the The safety of roundabouts, installed mostly in potential to increase safety when compared with France’s urban and suburban areas, including conventional intersections, just as they are residential areas, was generally superior to that projected to do in the United Kingdom. of signalized intersections.20 Researchers noted that large roundabouts with wide entries and Nevertheless, notwithstanding their good heavy bicycle traffic appeared to be less safe record, great care should be taken in layout than other roundabouts. In Germany the design to secure the essential safety aspects. number of collisions was 1.24 per 1 million The most common problem affecting safety is vehicles entering small roundabouts, compared excessive speed, both at entry or within the with 3.35 for signalized intersections, and 6.58 roundabout. The most significant factors for old traffic circles.20 In Norway an extensive contributing to high entry and circulating speeds collision analysis also revealed that are: roundabouts are safer than signalized intersections. The number of collisions resulting • inadequate entry deflection in injuries was 3 per 100 million vehicles • entering three-legged roundabouts and 5 per a very acute entry angle which encourages 100 million vehicles entering signalized three- fast merging manoeuvres with circulating legged (T-) intersections; it was 5 for four- traffic legged roundabouts and 10 for four-legged • (cross-) intersections (with and without poor visibility to the yield line signals).20 • poorly designed or positioned warning and In the United States, a recent study confirms advance direction signing the safety benefits of roundabouts. An • investigation of six sites in Florida, Maryland “Reduce Speed Now” signs, where and Nevada revealed that the conversion of T- provided, being incorrectly sited and cross-intersections (stop controlled and • signalized) to roundabouts decreased collision more than four entries leading to a large rates.20 According to the study, which was configuration sponsored by the Federal Highway Additionally, safety aspects to be considered in Administration, the reduction was statistically designing a layout include the following: significant. 1. Angle between legs: The collision potential Given that roundabouts have only recently of an entry depends on both the angle begun to appear in North America, roadway counter clockwise between its approach leg agencies have had little opportunity to gather and the next approach leg, and the traffic empirical data on the safety benefits of the flows. A high-flow entry should have a large structures. Fortunately, similarities between angle to the next entry, and a low-flow entry collision-prediction models developed in the

Page 2.3.12.4 December 2009 Geometric Design Guide for Canadian Roads

2.4.3 INTERCHANGE its capacity to collect and deliver traffic from the crossing arterial roads. LOCATION AND SPACING Interchange spacing in urban areas generally ranges from 2 km to 3 km. Interchanges should Rural freeways passing close to or through be located at major arterial roads, forming part communities require interchanges suitably of the arterial system of roads for the urban area located to serve the needs of the community. It and providing, or having the potential to provide, is sufficient to provide one interchange for small capacity to deliver to and collect from the communities; larger communities require more. interchanges. The precise location depends on the particular needs of the community; however, as a general Minimum spacing of interchanges is determined guide, interchanges should be located at arterial by the distance required for weaving (see roads recognized as major components in the Chapter 2.1), speed change lanes, and the road system, having good continuity and a appropriate placement of directional signs. capability for expansion if required. Interchanges should be located in the proximity If the arterial roads are spaced closer than 2 km, of major development areas; for example, it is usually necessary either to omit some of central business areas and areas of existing or the interchanges in favour of grade separations future concentrations of commercial or alone or adopt some alternative means of industrial development. As a general guide in combining interchanges to serve closely located rural areas, interchanges are normally spaced arterial roads. Figure 2.4.3.1 illustrates how this at between 3 km and 8 km. might be done. In the upper diagram the arterial roads are spaced at 2 km to 3 km, allowing On urban freeways, traffic conditions and driver each arterial to be served by its own behaviour and expectations are different from interchange. In the lower three diagrams the those of rural freeways, and this influences arterial roads are at less than 2 km, calling for interchange spacing. Operating speeds tend to some form of combined ramp / service road be lower, trip lengths shorter, traffic volumes system to provide the overall interchange ramp higher, and drivers are accustomed to, and capacity to serve the arterial system. The most anticipate the need for taking a variety of suitable configurations of ramps are very much alternative actions in rapid succession. site specific, and the design is dependent on Interchanges spaced at more than 3 km over a the layout of the arterial network and the length of urban freeway normally cannot provide particular needs of the community it serves. adequate service to urban development, and closer interchange spacing is called for. If Freeway collision rates tend to increase as 7 successive interchanges on urban freeways are interchange spacing decreases in urban areas. too close, however, the operation of the freeway This effect on collision rates is an important might become impaired and the freeway loses consideration in urban area interchange spacing.

September 1999 Page 2.4.3.1 Interchanges

Figure 2.4.3.1 Interchange Spacing on Urban Freeways

arterial roads at less than 2 km spacing, served by graded separated ramps

Page 2.4.3.2 December 2009 Geometric Design Guide for Canadian Roads

2.4.5 INTERCHANGE • traffic volume and traffic mix TYPES • number of interchange legs

2.4.5.1 General • traffic control devices

There is a wide variety of interchange types • topography available to the designer. The classification of the intersecting roads is a prime determinant in • right of way and property requirements the selection of the most suitable interchange type for any particular application. • service to adjacent communities Subsection 2.4.5.2 deals with interchanges between roads classified as freeways, either • systems considerations and design four-leg or three-leg. Subsection 2.4.5.3 consistency discusses interchanges between freeways and other roads, which are normally arterial roads • environmental considerations but in some cases are collector roads. Subsection 2.4.5.4 discusses suitable types for • economics interchanges between roads, neither of which is a freeway or an expressway. Such The relative importance of these controls and applications are normally between two arterial considerations varies between interchanges. roads or an arterial road and a collector road. For any particular site, each control is examined In rare instances, there is an application for an and its relative importance assessed. interchange between an arterial road and a local Alternative types and configurations are then road. studied to determine the most suitable in terms of the more important controls. While the Most interchanges provide for all movements selection of the best interchange type may vary between intersecting roads. Interchanges that between sites, it is important to provide regional provide a limited number of movements are consistency, where possible, in order to referred to as partial interchanges. For any reinforce driver experience. This would in turn movement provided for in a partial interchange, improve driver expectancy and hence safety. the corresponding return movement desirably should be available, since the driver expects to 2.4.5.2 Interchanges Between be able to retrace his route in the return direction Freeways on any particular trip. The absence of the return movement may even create a safety concern if Interchanges between two freeways are a frustrated motorist attempts a “wrong way” normally the most costly in terms of construction movement to regain access to a freeway. cost and property requirements.

The selection of the most suitable interchange Since freeways are fully-controlled access for any particular application, and the details of facilities, at-grade intersections within the its design, depend on a number of controls and interchange configuration are inappropriate and other considerations, among the most important it is mandatory that they be avoided. of which are: Fully directional interchanges provide for right • safety and left turns through large radius ramps having design speeds in the order of 70% to 80% of • functional and design classification of freeway design speeds and having overall intersecting roadways deflection angles in the order of 90o. Partially directional interchanges provide for some • adjacent land use left-turn movements by means of loop ramps, which have lower design speeds. Partially • design speed directional interchanges have applications

September 1999 Page 2.4.5.1 Interchanges where there are property limitations, or where 2.4.5.3 Interchanges Between some left-turn volumes are low. Freeways and Other Roads Four-Leg Interchanges In interchange types for this application, all Figure 2.4.5.1 illustrates fully-directional and ramps diverging and merging with the freeway partially-directional four-leg interchanges have acceleration and deceleration lanes so between freeways. The fully-directional type that traffic can enter and exit freeway lanes at, shown in illustration (i) provides single exits from or close to, freeway speeds. Where the ramps all four directions and directional ramps for all connect with the crossing roads, intersections eight turning movements. The through roads are designated with suitable traffic control and ramps are separated vertically on four devices. levels. Left-turn movements often are accommodated Partially-directional interchanges allow the on loop ramps, and loop ramps carrying traffic number of levels to be reduced as the number entering the freeway are preferable to loop of loop ramps is increased. The single-loop ramps carrying traffic exiting the freeway. Where arrangement, illustration (ii), and two-loop other considerations permit, it is desirable to arrangement in (iii) and (iv), require three levels. arrange the levels of the through roads so that A configuration in which loop ramps are carrying ramp traffic entering the freeway is on a the lighter volumes of left-turning traffic is downgrade and ramp traffic exiting the freeway preferable, and levels should be arranged so is on an upgrade, to assist in acceleration and that exit loop ramps are on upgrades deceleration respectively. encouraging deceleration, thus increasing safety. Illustration (v) is a full cloverleaf with The choice of carrying the freeway over or under collector lanes, incorporating loops for four left the crossing road depends on topography, cost turning movements and two levels separating and other environmental considerations. There the through roads vertically. This type of are a number of operational advantages in interchange introduces undesirable weaving carrying the freeway under the crossing road sections, and is only suitable where left-turn as discussed in Subsection 2.4.1.2. volumes are low and property is readily available. The collector roads are added to this Interchanges between freeways and other type of interchange so that the weaving categories of crossing road normally provide manoeuvres inherent in this interchange type for all turning movements. In some cases it may occur on the collectors rather than on the main be necessary to eliminate some turning line. movements; however, in such cases, for any movement that is provided the corresponding Three-Leg Interchanges return movement desirably should be available.

Figure 2.4.5.2 illustrates a variety of Figures 2.4.5.3 to 2.4.5.6 illustrate interchanges fully-directional and partially-directional for application between freeways and arterial three-leg interchanges between freeways. roads. They are shown diagrammatically with the arterial road crossing over the freeway and Illustrations (i) and (ii) are fully-directional with single exit ramps from the freeway. interchanges requiring three levels of roadways. Illustration (iii) is fully-directional, requiring only Diamond Interchanges two levels. Illustrations (iv) and (v) are partially-directional referred to as trumpet Figure 2.4.5.3 illustrates a Simple Diamond interchanges and each incorporates one loop interchange. The ramps intersect with the ramp for a left-turn movement. The choice crossing road at at-grade intersections between the two depends on the availability of controlled by traffic signals or, where volumes property, but desirably the loop carries the smaller volume of the two left-turn movements.

Page 2.4.5.2 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.4.5.5 Parclo B Interchanges

December 2009 Page 2.4.5.7 Interchanges

Figure 2.4.5.6 Parclo AB, Trumpet and Rotary Interchanges

• the weaving sections limit the speed and capacity • through crossing road traffic interrupted • complex signing • not suitable for future expansion • requires two wide structures • not in common use, hence low driver familiarity

Page 2.4.5.8 December 2009 Geometric Design Guide for Canadian Roads

Exit Terminal Length the speed is maintained until the end of the taper in the case of the parallel lane form or Table 2.4.6.2 shows design domain for lengths until the speed change lane has widened to of deceleration lanes in single-lane exits. For 3.5 m in taper form. The first concept, which parallel lane form, the length of deceleration provides the basis of the lower values of the design domain, assumes braking begins at lane, Ld is measured from the end of the taper the start of the parallel section (parallel lane (Lt). For the direct taper form, the length Ld is measured from the point at which the auxiliary form) or at the 3.5 m wide point (taper form) lane is 3.5 m wide. to decelerate to the control speed of the ramp. The second concept, which provides

The length of deceleration, Ld, is measured to the basis of the higher values of the design the beginning of the ramp controlling curve. If domain, assumes that the vehicle travels the ramp is relatively straight and ends at a stop for 2.0 seconds to 4.0 seconds in gear condition, as in a diamond interchange, the without braking followed by leisurely braking length Lt may be measured to the intersection. to the control speed of the ramp. This latter

Measurement of Lt and Ld are illustrated in condition is considered very generous, Figure 2.4.6.1. particularly in an urban situation. Two seconds in gear is assumed for roadway The length of an exit terminal is essentially design speeds up to 110 km/h; while determined by the distance required for 4.0 seconds in gear is assumed for deceleration after the vehicle has left the roadway design speeds of 120 km/h and through lanes. It is based on three factors in higher. combination as discussed in Subsection 2.4.6.2. They are: For two-lane freeway exits, the operation is different from the single-lane exit. This • the running speed on the through lanes operation may require a longer speed change lane than that for the single lane exit. The length • the control speed of the ramp proper is based on a different concept whereby the exit curve to the bullnose is considered to govern • the manner of deceleration the manoeuvre of the vehicle exiting to the left- hand lane of the two lanes. A total speed The design domain in Table 2.4.6.2 is based on change length (Ld) in the order of 400 m to the different assumptions applied to these three 450 m for mainline roadway speeds of 100 km/ factors: h or above is recommended by both AASHTO and the Geometric Design Standards for 1. The running speed on the through lanes is Ontario Highways for two-lane exits. assumed to be in the range of “operating speeds” used in Chapter 1.2. Where deceleration lanes are on grades equal or steeper than 3%, the length shown in Table 2. The control speed of the ramp proper at 2.4.6.2 should be adjusted by the appropriate the downstream of the exit terminal is grade factor shown in Table 2.4.6.3. included in the range of ramp design speeds in Table 2.4.6.1. The speed change lane Exit Ramp Transition Curve Criteria lengths corresponding to the ramp control speeds, which are governed by the design Interchange ramp alignments with their speed of the turning roadway curve, are relatively severe geometrics are particularly shown in Table 2.4.6.2. appropriate for the use of spirals. A spiral curve beginning in the vicinity of the bullnose is 3. The manner of deceleration includes a normally inserted between the exit curve or domain of two different concepts. For both taper ending at the bullnose, and the controlling concepts, it is assumed that drivers travel curve on the exit ramp as shown in Figure at the beginning of the speed change lane 2.4.6.2. (start of taper) at the operating speed and

December 2009 Page 2.4.6.7 Interchanges

Table 2.4.6.3 Grade Factors for Speed Change Lanes

DECELERATION LANES Grade Design Speed Grade Factor* of Roadway For All Turning Roadway Design Speeds (km/h) 6% to 5% up 0.8 3% to 5% up 0.9 less than 3% all 1.0 3% to 5% down 1.2 5% to 6% down 1.4

ACCELERATION LANES Grade Design Speed Grade Factor* of Roadway For Turning Roadway Design Speeds (km/h) 40 50 60 70 80 6% to 5% up 60 1.5 1.5 70 1.5 1.6 1.8 80 1.6 1.7 1.9 2.0 90 1.7 1.8 2.0 2.2 100 1.9 2.0 2.2 2.4 2.6 110 2.0 2.2 2.4 2.7 2.9 3% to 5% up 60 1.3 1.3 70 1.3 1.3 1.4 80 1.4 1.4 1.4 1.5 90 1.4 1.5 1.5 1.5 100 1.5 1.5 1.6 1.6 1.6 110 1.5 1.6 1.7 1.7 1.8 Grade Factor* For All Turning Roadway Design Speeds less than 3% (up or down) all 1.0 60 0.7 70 0.7 3% to 5% down 80 0.65 90 0.6 100 0.6 110 0.6 60 0.6 70 0.6 80 0.55 5% to 6% down 90 0.5 100 0.5 110 0.5

Note: * grade factor = ratio of length on grade tolength on level (as shown in Tables 2.4.6.2 and 2.4.6.5)

Page 2.4.6.8 September 1999 Geometric Design Guide for Canadian Roads ramp curve to the point at which the auxiliary Research has shown that safety can be lane is 3.5 m wide. Acceleration lengths, La, are increased on acceleration lanes with increased measured from the end of the ramp controlling length, especially for high speed facilities. curve. Measurement of Lt and La is illustrated in Figure 2.4.6.1. Where the entrance terminals occur on a crest curve, sight distances to the lane drop may be The length of an entrance terminal is based on affected, longer acceleration lanes will then be three factors in combination as discussed in required as discussed later in the ‘Sight Subsection 2.4.6.2. They are: Distance’ Subsection.

• merging with the through traffic Entrance Ramp Transition Curve Criteria

• control speed of the ramp proper In entrance ramp design, a spiral is introduced in the vicinity of the bullnose between the ramp • manner of acceleration controlling curve and the entrance curve or taper to effect a smooth transition. Acceleration starts The merging speed with the through traffic is at the beginning of the spiral, and usually assumed to be the range of operating speeds continues beyond the spiral on the ramp used in Chapter 1.2. terminal section. The radius of the spiral increases so as to accommodate the increasing The control speed of the ramp proper at the speed of the vehicle accelerating. The spiral upstream of the entrance terminal is included in parameter needs to be small enough to provide the range of ramp design speeds in Table 2.4.6.1. a sufficiently rapid rate of increase in radius to The speed change lane lengths corresponding match the acceleration of the vehicle. On the to the ramp control speeds, which are governed other hand the spiral parameter needs to be by the design speed of the turning roadway sufficiently large to ensure that the comfort, curve, are shown in Table 2.4.6.5. superelevation and aesthetic criteria are met. The spiral parameter, can be selected from the The manner of acceleration assumes an increase range given in Table 2.4.6.6. of speed based on the acceleration of passenger 3,14 cars tested in the U.S. Table 2.4.6.6 Spiral Parameter for Where acceleration lanes are on grades steeper Entrance Ramp than 3%, the length shown in Table 2.4.6.5 should Transition Curves be adjusted by the appropriate grade factor in Table 2.4.6.3. Ramp Controlling Design Domain of Curve Speed Spiral Parameter The length of an entrance terminal also depends (km/h) (m) on the relative volumes of through and entering 40 50-80 50 65-130 traffic. Longer entrance terminals (i.e. the higher 60 85-140 values of the design domain in Table 2.4.6.5) are 70 110-280 desirable on higher volume roads to enable 80 125-360 entering traffic to merge with through traffic safely and conveniently. Sight Distance at Entrance Terminals Trucks and buses require longer acceleration lanes than passenger cars. Where a substantial At entrance terminals, the driver is looking for number of large vehicles entering the road is a gap in the traffic in the adjacent lanes in order expected, longer acceleration lanes are to effect a lane change and merge. A driver appropriate. therefore has to look back to find an appropriate gap. This view is best provided by maintaining the vertical alignment of the ramp in the vicinity

December 2009 Page 2.4.6.13 Interchanges of the nose at elevations similar to, or above, on a through lane to prepare for the merge ahead those of the through road. If the ramp is after passing the exit nose. significantly lower, the driver might have some difficulty effecting a safe merge. If the ramp is Figure 2.4.6.5 shows the minimum values for higher, the driver normally has good visibility ramp terminal spacing based on design speeds. unless the view is obscured by a traffic barrier Additional distances may be required to ensure or other visual obstruction. signing requirements are met.

A driver begins accelerating from the ramp 2.4.6.6 Safety and Design controlling circular curve some distance before Overview the nose, usually in the vicinity of the beginning of the spiral curve. At this point, the driver looks The fundamental principle of an interchange is for a gap in the stream of traffic in the adjacent the movement of vehicles through the o lane. The line of sight is taken to be at 120 interchange in the safest, most efficient manner from the direction of travel and the object to be possible. The ability of an interchange to seen is taken to be in the centre of the adjacent accommodate drivers in this manner is closely lane, 1.0 m above the pavement surface. This related to the efficiency with which the is illustrated in Figure 2.4.6.4. information is provided to the driver and with the degree to which driver expectancy is met at To allow the driver to make a merging the interchange. manoeuvre safely, ideally he requires a view of the entire speed change lane at the nose as Interchanges present the motorist with a illustrated on Figure 2.4.6.4. The driver may not complex set of decisions that require quick have this view if the speed change lane occurs evaluation and action. Designers can reduce on a crest curve, in which case the vertical drivers’ stress at interchanges by keeping the alignment should be adjusted so as to shift the alignment simple and direct, maintaining design crest curve away from the speed change lane. consistency, providing sight distances greater If this is not feasible, either the speed change than the minimum stopping sight distances, and lane should be lengthened or the crest curve using above minimum design criteria for other should be flattened to provide, preferably, geometric elements. decision sight distance to the end of the taper as discussed in Chapter 1.2. Collisions on ramps and connecting roads generally increase with traffic volume and with 2.4.6.5 Ramp Terminal Spacing decreasing curve radius.7 It also appears that upgrade exit ramps have lower collision rates Successive ramp terminals on freeways/ and thus it is preferable, from a safety view expressways or within an interchange are point, for the connecting road to pass over the spaced to allow drivers to make decisions in freeway or higher speed road. The use of sufficient time to make safe manoeuvres. collector lanes for high volume interchanges enhance safety, especially where loop ramps In the case of successive exits, the distance is are used7. The use of a collector introduces an based on the provision of adequate signing. In intermediate-speed facility between the freeway the case of successive entrances, the length is and the off-ramp thereby encouraging speed based on the merging manoeuvre length slow-down prior to entering the off-ramp. required for the first entrance. One of the key issues related to interchange An entrance followed by an exit terminal creates design involves heavy truck incidents at a weaving condition, and is discussed in interchanges. In general, tight radius curves Chapter 2.1. on ramps and short speed change lanes cause problems with heavy trucks. Truck incidents The distance between an exit followed by an on interchange ramps generally involve loss of entrance needs to be sufficient to allow a vehicle control leading to rollover or jack-knife. Recent

Page 2.4.6.14 September 1999 Geometric Design Guide for Canadian Roads

2.4.7 OTHER 2.4.7.2 Ramp Metering

INTERCHANGE Ramp metering consists of traffic signals DESIGN FEATURES installed on entrance ramps in advance of the entrance terminal to control the timing and 2.4.7.1 Operational Analysis number of vehicles entering the freeway. The traffic signal may be pre-timed or traffic- A series of proposed interchanges or a actuated to release the entering vehicles proposed single ramp exit/entrance can be individually or in platoons. subjected to an operational analysis, including capacity and design features, after the planning The purpose of ramp metering is to reduce or preliminary design stage. The operational congestion or improve merge operations on analysis is basically a test for ease of operation urban freeways. This strategy has been proven and for route continuity from a driver’s point of effective in many applications throughout North view, both of which are affected by the location, America. Ramp metering can optimize freeway proximity, sequence of exits and entrances, vehicle flows and improve merge area safety merging and diverging movements, necessary through uniform spacing of entering vehicles. weaving, practicability of signing, visibility of the target destination, and clarity of paths to be The ramp metering system can include the followed. The operational analysis can be following: considered as an application of the ‘driver • workload’ approach to the evaluation of design stop bar marking upstream of the ramp consistency. bullnose • A route may be tested by isolating a single path passage loop downstream of the stop bar of travel and examining it only with regard to to monitor vehicles released from the queue other features of the layout that will affect a • driver on the path being tested. The test can ramp control signs be made using an overall plan on which the • number of traffic lanes, the peak hour volumes, demand detector loops upstream of stop the expected running speeds, the visibilities of bar downstream features, and the signing are • shown. upstream queue detector on the ramp immediately downstream of the arterial The operational analysis indicates whether or street bullnose not confusion is likely due to the close proximity • of exits and entrances, or traffic conflict is likely freeway detector loop on the upstream because of weaving movements. It should approach to determine available gaps illustrate also the clarity of the path and the Provisions for ramp metering bypass lanes may feasibility of signing. The test may show that be required for potential implementation of high the path is easy to travel, direct in character, or occupancy vehicle priority features. The it may show that the path is sufficiently complex suitability of ramp configurations to and confronted with disturbing elements that accommodate bypass lanes hinges on the require adjustments in design. availability of required length along the ramp. The operational analysis can also include a check of the design adequacy, e.g. deceleration 2.4.7.3 Bus Interface length and ramp radius, in conjunction with the test for ease of operation. This is a particularly Accommodating transit on a freeway can lead useful approach in checking the overall design to a high level of transit service in larger cities. of interchanges that have non-typical An interchange can provide a transfer point configurations. between freeway bus service and local bus

September 1999 Page 2.4.7.1 Interchanges feeder service and, potentially, commuter to perceive gaps in the traffic flow. At ramp parking facilities. crossings where there are insufficient gaps in the traffic flow to allow pedestrians to cross, Design considerations of bus interface at pedestrian-actuated signals or a pedestrian interchange includes the following: overpass/underpass can be considered. In general, it is desirable to provide pedestrian • safety crossings in the shortest distance feasible.

• capital costs The accommodation of bicycles at interchanges is discussed in Chapter 3.4. • alignment and geometrics 2.4.7.5 Grading and Landscaping • intended operation with respect to interface Development location, platform facilities and local bus facilities Grading at an interchange is determined mainly by the alignments, profiles, cross-sections and • impact on crossing and bus road drainage requirements for the intersecting roads access /egress locations and ramps. Contour grading can be designed to increase safety and enhance aesthetics. Flat Sufficient sight distance on the bus route is slopes should be used where feasible, to important for maintaining safe and efficient increase safety and to enhance the appearance operation of the buses. Interfaces at of the area. V-ditches and small ditches with interchanges should be located as closely as steep side slopes are usually avoided. Drainage practicable to crossing roads of suitable channels and related structures should be as geometrics and ridership potential. Pedestrian inconspicuous and maintenance-free as walk distances should be minimized between feasible. The need for a ‘forgiving’ roadside on local and freeway buses. the outside of curves is particularly important for tighter interchange ramps. On diamond-type ramps, the bus interface may consists of a widened shoulder area adjacent Contour design is usually applied to residual to the ramp roadway or may be on a separate areas of land in interchange areas between road. Generally, bus interface provided on ramps to create noise berms and for material entrance ramp is preferred. Bus interfaces are disposal areas. These areas can be graded more difficult to provide effectively within with varying slopes to give an undulating and cloverleaf or directional-type interchanges. 4 natural looking appearance. Contour design is Guidelines have been developed for use in carried out in conjunction with drainage design preliminary design of Parclo A interchanges with consideration for safety and, where where bus interfaces are to be protected. appropriate, in conjunction with landscaping. Typical bus interfaces at Diamond interchange Residual pockets of land in interchange areas, and at Parclo A interchange are shown in particularly loop ramps can be used to dispose Figure 2.4.7.1. of surplus material and to minimize spoil. Conversely, they may be used to generate 2.4.7.4 Pedestrians additional excavation and to minimize borrow. In placing fill in loop ramp areas, care must be The accommodation of pedestrians through an taken not to compromise sight distance urban interchange should be considered during requirements. the development of interchange configuration. Grading of areas on the back slope can be In addition to allowing the driver to detect the contoured to match the adjacent topography presence of pedestrians, adequate sight and can greatly improve the aesthetics of the distance for the pedestrians must be provided road. This can be accomplished, in part, by where pedestrians are expected to cross an rounding the edge of the interface between the interchange ramp. Pedestrians must be able cut of slope and the adjacent hillside, and

Page 2.4.7.2 December 2009 Geometric Design Guide for Canadian Roads

Figure 2.4.8.6 Typical Design Entrance Terminal Parallel Two Lane class

December 2009 Page 2.4.8.7 Interchanges

Figure 2.4.8.7 Typical Design Entrance Terminal Tapered Single and Two Lane

Page 2.4.8.8 September 1999