GUIDE TO THE SE.LECTION
OF NONDIRECTIONAL INTERCHANGES
By
Charles Pinnell James D. Barnett John Haynes Milton Radke
Cooperative Research with the Texas Highway Department and the
Department of Commerce I Bureau of Public Roads
Final Report on Research Project 2-8-58-6 Ramps and Interchanges
November, 19 63
TEXAS TRANSPORTATION INSTITUTE Texas A&M University
College Station I Texas ,, GUIDE TO THE SElECTION OF NONDIRECTIONAL INTERCHANGES
INTROpUCTION
The research project "Ramps and Interchanges" was initiated September 1, 19 58 with the objective of developing design criteria for entrance and exit ramps and interchanges and to correlate design factors and operational characteristics of freeway interchanges. The research work has been con centrated in three basic areas which are as follows:
1. Freeway Entrance and Exit Ramps.
2. Diamond Interchanges.
3. Cloverleaf Interchanges.
Research publications which have been developed from the Ramps and Interchanges project are as follows:
1. "Freeway Ramps;•: Report 1960. Charles Pinnell and C. J. Keese.
2. "Driver Requirements in Freeway Entrance Ramp Design;•: Proceed ings, Institute of Traffic Engineers, 1960, Charles Pinnell.
3. "Traffic Behavior and Freeway Ramp Design," Proceedings, ASCE 1 No. HW3, September, 1960. Charles Pinnell and C. J. Keese.
4. "Capacity Study of Signalized Diamond Interchanges I" Highway Research Board Bulletin 291, 1961. Donald G. Capelle and Charles Pinnell.
5. '"Design and Operation of Diamond Interchanges," Research Report1 19 61. Charles Pinnell and Donald G. Cap~lle.
6. "Operational Study of Signalized Diamond Interchanges,'' Highway Research Board, 1962. Charles Pinnell and Donald G. Capelle.
7. "A Study of the Peaking Characteristics of Signalized Urban Intersections as Related to Capacity and Design," Highway Research Board, 1962. Donald R. Drew and Charles Pinnell.
8. "Design and Signalization of High-Type Intersections, .. Research Report, 1961. Donald R. Drew and Charles Pinnell. 9 . 11 The Value of Signal Phase Overlap in Signalized Intersection 11 Capacity , Traffic Engineering 1 December, 1962. Charles Pinnell.
10 . 11 At-Grade Intersection Spacing on Conventional Two-Level Diamond 11 Interchanges, Research Report I 19 63 1 James D. Barnett and Charles Pinnell.
ll . 11 A Comparative Study of the Conventional Diamond and Cloverleaf 11 Interchanges with Respect to Ramp Capacity and Vehicular Delay I Research Report, 1963. James D. Barnett.
The purpose of this final report is to present data from a comparative study of diamond and cloverleaf interchanges with a summ?ry of previous research findings as to provide a guide to the selection of nondirectional interchanges.
BASIC COMPARISONS
Interchange Types
After considering the various types of nondirectional interchanges, the following three interchange types were selected for comparative study:
1. Conventional Two-Level Diamond .
2. Cloverleaf ·without Collector - Distributor Roads.
3 . Cloverleaf with Collector-Distributor Roads.
It is recognized that there are numerous variations of the above inter changes such as the split-diamond I three-level diamond I and partial cloverleaf but it was felt that basic studies of the above interchanges would pro:V!ide data applicable to the different variations.
Comparative Factors
Studies were initiated to compare the diamond and cloverleaf inter changes with regard to the pasic factors of (1) capacity, {2} efficiency 1 and {3) cost.
Specific data on these factors considering both the diamond and -clover leaf interchanges will be presented in the following material.
2 RESEARCH STUDIES .. Research studies on the "Ramps and Interchanges 11 project were conducted in a number of locations. Some of the principal study locations were as follows:
1. Wayside Drive Interchange - Gulf Freeway Houston. Diamond inter change capacity studies. Figure 1.
2. Cullen Blvd. Interchange- Gulf Freeway Houston. Diamond inter change capacity studies. Figure 2.
3. Marshall Street On-Ramp - San Antonio. Ramp capacity and delay studies. Figure 3.
4. Berry Street Interchange - Fort Worth. Capacity 1 signalization and delay studies on diamond interchange. Figure 4.
5. North Central Expressway - Loop 12 Interchange Dallas. Cloverleaf capacity and delay studies. Figure 5.
6. Griggs Road Entrance Ramp - Gulf Freeway Houston. Capacity and delay studies. Figure 6.
Project studies were conducted at the above locations during the period September 1958 to September 1963 to develop data on the factors indicated. Data reported in this report were collected at locations listed as numbers 3, 4 and 5.
INTERCHANGE CAPACITY
Diamond Interchange
A procedure for determining the capacity of a diamond interchange was developed in an early phase of the research work. This procedure is explained and illustrated in references 4 and 5 on page 1.
The capacity of a conventional diamond interchange (Figure 7) is determined in terms of the number of vehicles able to move through the signalized at-grade intersections. The "critical capacity" of a diamond interchange, defined as the sum of the largest single lane volume on each of the four approaches as shown
in Figure 8 1 can be as large .as 1760 vehicles per hour when proper signalization is utilized,
If a total of three lanes is assumed on each of the four approaches to the diamond the following total capacity is available:
Total Capacity= 3 x 1760 = 5280 vph
3 WAYSIDE DRIVE INTERCHANGE HOUSTON, TEXAS FIGURE I "
CULLEN BLVD. INTERCHANGE HOUSTON, TEXAS FIGURE 2 (_ FRONTAGE ROAD ------
FREEWAY -•• ..• - - ~ m - ]._ ------~------DRAINAGE CREEK 115' - - - --
h,------_/_------
lb: ~ 2~' t ~..,... - 36' ' - ...... - - + FREEWAY ~ =- .. • • .. ! • • - FRONTAGE ROAD ----, ------
MARSHALL STREET ENTRANCE RAMP FIGURE 3
'"' 0
I I 20' I ~ I I I I I I Io.s· I I ~ I i~ I ~ I I """'I I I , I 18' R. l~~R. I~ ~ 10' R. J ~ ------. I I I I ' I ' ' ·o I' ' ' ' ' I ' ' CD I I I ' I ' I I I "' I I r-r-·~·----·! I I "' I I ' . ' Inc:: I I I I I I I I ' I ' I u I I ~ I I I I I I I I ' I 0 ~··· I I "' I ' ' I ' ___!r ' ' Tr:;:------~- --- (< ')~ 18 R. . I 18' R. 18' R. I 32' I I I ~ -., I 3: G) LLJ LLJ 0:: u..
I cri == 0 I I I I :Z I I' I I I H j BERRY STREET INTERCHANGE FORT WORTH, TEXAS FIGURE 4 LOOP 12-NORTH CENTRAL EXPRESSWAY INTERCHANGE DALLAS,TEXAS
FIGURE 5 ,_ FRONTAGE ROAD ~ ~ -- --.--• ------· --+TO HOUSTON US 75 _ __,__FREEWAY __ -..,.------,------.------_f ____· _· - ---- f --.- ·..:_· _..;...,.. 36' 46' FREEWAY • -----t------~-.-.- ~ -- --u += _· - . ~-'------ r------~--- 650' -~-'------,-. ------33' -- . --- t FRONTAGE ROAD ' f GRIGGS ROAD ENTRANCE RAMP · GULF FREEWAY HOUSTON FIGURE 6 I 0z 0 ~ <{ 0 w _J ~ <{ <{ ['-. ~ n ~ I- 0:: S2 z w 1=4 w I >z z 0 (.) w I I- C· ~ lll FREEWAY FRONTAGE FRONTAGE ROAD ROAD I I I I I I . I I I I I I 0 t I I •CD-. - -~ - - - -· -- - - I ® I I ' I I I I· I : I I CRITICAL FLOW APPROACHES ON SIGNALIZED DIAMOND INTERCHANGE .F«S.UR:E a It is not possible to set ?- capacity value which will fit all diamond interchanges as the capacity will vary depending upon the number of lanes available on each approach. Thus, it is necessary to utilize the capacity evaluation procedures to study the capacity of any given diamond inter change. Attention should be called to the fact that the "critical capacity" is related only to that traffic passing through the signalized intersections. Special right turn lanes such as those shown in Figure 9 can divert traffic around the signalization and greatly increase the capacity of the diamond interchange . Cloverleaf Interchange In the case of the cloverleaf interchange it is not possible to define its capacity in terms of total interchange volume as in the case of the diamond interchange since the movements are not restricted by traffic signals. Traffic studies indicate, however that the basic capacity limitation of the cloverleaf interchange without collector-distributor roads is on the left turn movement either off of or on to the freeway. This move ment passes around a loop and enters the freeway or major street through an entrance ramp. This movement is restricted to one lane and the ramp geometries are less than desirable due to the short loop radius. The left turn movement on a cloverleaf without collector-distributor roads was studied in detail at the study site shown in Figure 5. From analyses of the traffic data the loop capacity curve shown in Figure 10 was developed. The loop capacity is directly related to the volume in the outside lane of the freeway and drops to less than 600 veh per hour with freeway volumes in the outside lane greater than 1200 veh per hour. For a cloverleaf with a collector-distributor road as shown in Figure 11, the loop capacity for major arterial to freeway interchange is not dependent upon freeway volume . The loop traffic enters the collector distributor road from the loop and interchanges with the freeway through a regular frontage road entrance ramp. However, for traffic interchange from the freeway to the major arterial, the capacity curve shown in Figure 10 is applicable. Due to the improved geometries that can be obtained in the design of the frontage road to the freeway ramp, increased capacity can be obtained. Studies of this type ramp were conducted at the study 12 (. ,t tl ~c<:::::' dI III GI ~~ I II I I II I II_ • 1 ------.. ------.- 1 II I I II I FREEWAY I II I ~s=:= Jit .~~ FRONTAGE ROADS I I I I I I I ~I I I I I I I I I I I I I I , SPECIAL RIGHT TURN LANE FOR CONVENTIONAL DIAMOND INTERCHANGE FIGURE 9 25 20 • -:IE ~ 15 -UJ :E ;:) g..;.1 a. 9 10 • 5 I 0 5 10 15 20 25 FREEWAY LANE VOLUME (VPM) PLOT OF OUTSIDE FREEWAY LANE VOLUME VERSUS CLOVERLEAF LOOP VOLUME AT OR NEAR CAPACITY FIGURE 10 i0 site shown in Figure 3 and the capacity curve shown in Figure 12 was developed. This ramp capacity was approximately 300 veh per hour higher through the range of the freeway volumes than that of a direct loop ramp. Utilizing the previously presented curves and considering specific design volumes 1 it is possible to make a capacity analysis of any .given cloverleaf interchange. INTERCHANGE EFFICIENCY The basic consideration in efficiency is the delay encountered by traffic in passing through an interchange. In order to compare this delay a study of specific movements was conducted. The traffic move ments were considered in the two main categories of (1) traffic moving from the major street to the freeway and (2) traffic moving from the freeway to the major street. After considering both general movements it was decided that the traffic movement from the major street to the freeway was the most critical for comparison and was further broken down as follows: Movement A--Traffic passing through the interchange. This is non interchanging traffic and usually makes up only 20-30 per cent of the total traffic in the interchange area. The cloverleaf interchange causes only slight delay to this traffic due to marginal friction. Due to the signalization I the diamond interchange will stop a por tion of this movement. Movement B--Traffic turning right onto the freeway, This movement can be handled by right~turn loops or special right turn lanes on both the diamond and cloverleaf interch9nge and the delay is essentially the same for both types of interchanges. Movement C--Traffic turning left onto the freeway. This movement encounters delay on both types of interchanges. After considering all three movements it was decided that the most critical movement to compare was movement C 1 and specific studies were conducted to study this movement on both the cloverleaf and diamond interchange. 16 Q 30 25 • • • • ~----G--1...... ®- • • • ...... ®._ • • • 20 • ...... ®... - ...... 0..... • • • 'e, 'e...... • . . 0 . 2 • ''e, )Y =21.4 7-0.013 X 'e...... • ...... e, • ...... • ...... '®...... 't, • • ...... 10 '<> 51-- I I I I 0 5 10 15 20 25 30 FREEWAY VOLUME (VPM) PLOT OF OUTSIDE FREEWAY LANE VOLUME VERSUS DIAMOND ON-RAMP VOLUME AT OR NEAR CAPACITY FIGURE 12. Diamond Interchange Travel Time Motion picture studies were conducted at the Berry Street Interchange shown in Figure 4 to obtain data on the average travel time for a vehicle to· pass through the signalized intersections 1 turn left and move onto the freeway (Figure 13}.. This travel time was broken down into three basic .parts as follows: 1. Time to move through the at-grade intersections. 2. Time to move from at-grade intersection to ramp entrance. 3. Time to move into the freeway from the ramp. It was found that the time to move through the signalized inter sections was largely dependent upon the cycle length of the signals. The cycle length in turn was found to be related to the total intersection volume as shown in Figure 14. Thus as the intersection volume in creased I the cycle length increased and the travel varied as shown in Figure 15. Substantial delay was encountered when cycle lengths increased to 8 0 or more secpnds in length. The time to move from the at-grade intersection to the ramp entrance was a function of the ramp location and was computed assuming a speed of 30 mph. The time to move from the ramp entrance into the freeway was deter mined from studies of entrance ramp operation. A total entry time was determined by combining the average travel time for ramp vehicles under free flow conditions to move into the freeway with an average delay time at entrance ramps. The average free flow time for ramp entrance was found to be 13.4 seconds and average delay times were developed as shown in Figure 16 . From the curves and data previously mentioned/ it is possible to fix the average travel time for a left turning movement on a diamond interchange. The following are examples of this determnation: Example 1 Cycle Length ll5 sec. Freeway Flow in the Outside Lane 1200 veh/hr Left Turn Volume 600 veh/hr 18 v t1II II ill II II - II II !. II II '· II I I I I ..' I I f· I I I I h I I "m I I 600' = I I H· I I I I ' I I I I I I I ' I I I I I ".. I I I ~ I 3=; I \\ w: LLJ· a: I~I ' LL I \ I I ' I l I I I I 100' : I I I~ I I - I I ARTERIAL STREET DIAMOND AND CLOVERLEAF LEFT-TURN MOVEMENTS FIGURE 13 I I 120 I I I I I I / I 110 / • 0.92 I I (i) I Q LU I z ..J / 0 u I [rl II) ffi1oo 1/ 0.89 0: ~ LU I .o.. ~ / u // . 13 > ..J (.) /• u 0: / i: LU /" D.. 90 . 0.96 ~- LU 1.1. :::E LLt ;::1 :::E ..J ;::1 g •• // 19.48+ 0.0063(X)2 ..J ~Y= 0 z ;'• > Q . z 1- 80 / u •/ 0.82 ~ LU (I) / u 0: / . LU II) LLt /. a:: / LU ~ ,.I ..J ~ < .. ..J ~ 70 . 0.78 <( ~ 60 0.75 50L___----·--::-!=------=.::------:~----::-';:-----=----:-":::-----=-....!o~.7u2w ~ oo ro w oo ~ oo ~ CYCLE LENGTH (SECONDS) PLOT OF CYCLE LENGTH VERSUS TOTAL INTERSECTION VOLUME PER CYCLE FIGURE 14 • 80~ /" / / // / / / / 70 /" / . / / / / / / / / " / . / ///~Y=-32.30+0.93X •/ /' / / / / / / / ./ ~ / 40 / / /' / / / :// 30~------~------~~------~------~~------~------~--~ 60 70 8 0 90 100 110 120 (CYCLE LENGTH- SECONDS) 0.73 0.76 0.79 0,83 0.87 0.92 . (INTERS.ECTION VOLUME-VEHICLES PER SECOND) 51 61 71 83 96 110 (INTERSECTION VOLUME- VEHICLES PER CYCLE) PLOT OF WEIGHTED AVERAGE TRAVEL TIME FOR INSIDE LANE OR LEFT TURN LANE QUEUES FIGURE 15 15,-- ...... 5.2 ...... 13.2 ' . ' ' ' ' 8.5. ' ...... ' ' ' ' ' ...... ' ' ' 1.9. \. ' ' ' " \ ' "' '{~·8 ' ...... 1.5. \. \ ...... " \. 4.5 \ \ 101-- ' \ . ' ' \ \ 4.9\ \ eQ. " \. 1.8 ..:: ' . \ \ 11.6 \. . \ w \ \ :::!: 1.6 1.3 \ 2.4 \ ::> \...... J \ \. \ 0 > \ \ \ \ Q. \ :::!: \ \ 0 o 5 10 15 20 FREEWAY VOLUME (vpm) AVERAGE DELAY ON DIAMOND RAMP FIGURE 16 ·' Travel Times (a) Through signals 75.0 sec. (b) · Time to Ramp Entrance 3.4 sec. (c) Time to enter Freeway · 25.4 sec. Total 103.8 sec. Example 2 Same traffic but utilizing a 70 sec. cycle Travel Times (a) 32.0 sec. (b) 3. 4 sec. (c) 25.4 sec. Total 60.8 sec •. The effect of the cycle length is demonstrated by the previous ex amples. The long cycle lengths observed in the Berry Street Study indicated that the completely efficient overlap phasing was not being obtained. With proper overlap phasing it would be possible to carry the traffic volume encountered on a 60.,-80 seconcl cycle, An earlier operational study conducted at the Berry Street Interchange (Report Number 5) offers evidence that it was possible to operate the interchange efficiently on a 60-80 second cycle. The data listed below were taken from this study and show the variation in cycle length obtained Berry Interchange Data Period - 4 p.m. to 5:20 p.m. Study System Average Interchange Cycle Length Total Volume I 3-Phase 118 3856 II 4-Phase 105 3930 III 4-Phase/Overlap 77 3825 IV Fixed Time 80 3778 23 from various phasing arrangements. With the proper phasing and signal equipment adjustment, however, it was possible to operate satisfactorly with an 80 second cycle or less. A further illustration of the ability to carry large volumes with a 60-80 second cycle was reported in a recent California study. 1 This study involved conventional diamond interchanges whose signalization utilized the recommended phasing and which operated on a 60 second cycle. The volumes moved on this 60 second oycle are shown in Figure 17 and compared with the Berry Street volumes. The report in dicated that peak hour traffic at these interchanges moved with no apparent congestion and with small qUeue lengths. In summary it can be stated that if efficient phasing is utilized, cycle lengths of 60-70 seconds are satisfactory and that the average delay to a left turning vehicle approaching a diamond interchange will be on the order of 50-60 seconds during peak periods of traffic flow. 1 "Signalization of Diamond Interchange" by Salem Spitz in April 1964 issue of Traffic Engineering. 24 ------ " FREEWAY I I I I "' 600 _j t I I I I I I I L ------.1400 s sssssssgsssssssssssssss~ 1500.. ------ I I CR1TICAL 600 LANE J1400 900 " VOLUMES 1500 • 900 l I I TOTAL 4400 I I ~ CALIFORNIA DATA CYCLE LENGTH FOR Fl XED TIME CONTROLLER = 60 SECONDS FREEWAY I I I I _j 334 I I I I I I I :. • I L I ------.1000 05 s s s SS s s 5 s 938. ------ I I CRITICAL} 334 860 LANE 938 • VOLUMES 1000 I I 860 I I l TOTAL 3132 ~ BERRY DATA AVERAGE CYCLE LENGTH FOR ACTUATED CONTROLLER= 109 SECONDS VOLUME AND CYCLE LENGTH COMPARISONS CONVENTIONAL DIAMOND INTERCHANGES FIGURE 17 Cloverleaf Interchange Travel Time Studies of the left turn movement on the Cloverleaf interchange (Figure 13) were conducted at the site shown in Figure 5. The clover leaf studied did not have a collector-distributor road. The travel time for the left turning traffic was found to be related to the following: 1. Rate of flow on the outside lane of the freeway. 2. Rate of flow of left turning vehicles. 3. Loop radil.ls. Since the vehicles using the loop must merge with the outside lane of the freeway, the capacity of the weaving section becomes very important in any analysis. Since the freeway vehicles have the right of way in the merging area it may be expected that travel time on the loop increases with an increase in the outside freeway lane volume. This is due to a reduction of acceptable gaps in which to merge and a reduction in the loop capacity. This has been shown in Figure 10. When the rate of flow into the loop exceeds the loop capacity, stack-up occurs. Thereforel the amount of stack-up, or the number of vehicles already in the loop, exerts some influence on travel time. The radius of the loop studied was 100 feet. Under free flow condi tions, travel time on the loop was found to be an average of approxi mately 22 seconds. As a result, the outside freeway lane volumes, the corresponding loop volume, individual travel times for loop vehicles, and accumu lation of stack-up were removed from the film on a per minute basis. It was not possible to study the effect of a larger loop ra,dius since only one study site was available. A multiple regression was then run on the collected data. This regression produced the following equation: 2 Y = 26.68- 0.8758 (V 1) + 0.079 (V 1) + 2.448 (s) where Y = average travel time (sec.), V 1=,freeway outside lane volume (vpm) 1 and s = accumulation of vehicles on the loop at the end of the previous minute. The regression indicated that the above equation would account for approxi mately 67 percent of the factors influencing loop travel time. It should be noted that loop volume affects loop travel time through the accumulation of vehicles on the loop. Using the above equation, a family of curves to determine loop travel time was developed and is shown in Figure 18. Use of these curves, by neces sity (because of stack-up accumulations) becomes an iterative procedure. This procedure is as follows: 1. Determine rate of flow in outside freeway lane (vpm). 2. Determine rate of flow into loop (vpm). 3. Consult Figure 10 to determine if the loop capacity has been exceeded. 4. The number of vehicles by which the loop capacity has been exceeded plus any stack-up not cleared from previous minutes is the amount of stack-up at the end of each minute. 5. Using the appropriate stack-up curve the average travel time is determined for that minute. 6. The procedure is then repeated for the length of the study period. It can be observed from the curves in Figure 18 that travel time on the cloverleaf ramp increases rapidly when the left turning traffic exceeds the loop capacity. An example of the travel time for a left turning movement below the loop capacity is given below. Example (same conditions as examples for diamond interchange) Freeway Flow outside lane 1200 - veh/hr Left turn volume - 600 veh/hr Travel Time (a) To loop entrance - 8.5 seconds (b) Loop travel time -41.0 seconds (c) To end of acceleration lane -13.6 seconds Total 63. 1 seconds 27 160 I• I I I • .. I / I 1 • 140 I I I • I I I 1 I • I 1 I I I • I I I I I I • 1 i20 I I I I I I • I I / I I I I • ////// (.)w I • I I I I !!! I /llll .100 • I I • I I I • . I I ~ I I I I I 9 / I I I I I u. ,_o/ I • I • I / <( ~rt/ 1 I I I 1 UJ _. c.~/ 1 .1 I 1 1 "--""/ ffi eo <:>/ rq/ / ··I • I • +'• . I I I / .;:,q/ I I I I ~ __.,. c.""/ I ------.... "-'If/ / • I I d ""~ ,'V . I • ,• .;:,q'/" / 1 1 ~ ,..,."' c.*'/' / 1 I ------...... 0"--""/ • I w 60 1 'bl I ~· .;:,q"!" / I i=• ,.,"' v~/ / I ------•.,...... -<...'~-/ / I• _. ""/ I>< UJ 1 • ~ ../ I PROOEOURE FOR USING CURVES! ~ _..,. c,*-~ I KNOWING LOOP 8 FREEWAY ---•- "--""/ I OUTSIDE LANE VOLUMES, ~ ""/ 0/ CONSULT FIG. 10 TO DETERMINE • q~ • IF RAMP CAPACITY HAS BEEN 40 / *"..:> / _.."' ..,.c./ EXCE~DED. W SO STACKOP "--/ EQUALS NUMBER ABOVE CAPACITY. ---- __ ...... USING THE APPROIATE STACKUP .""/ CURVE THE AVERA.GE TRAVEL / . TIME FOR THE NEXT LOOP _.. / ------·- VOLUME CAN BE DETERMINED. 20 TRAVEL TIME• 26.167- 0.7218 (FREEWAY LANE VOLUME)+ 0.07304 (F~EEWAY LANE VOLUME)+ 2.271 ( STACKUP) 0 o 10 20 30 40 RATE OF FLOW IN OUTSIDE LANE OF FREEWAY (vpm) PLOT OF FREEWAY RATE OF FLOW IN OUTSIDE LANE VERSUS LOOP TRAVEL TIME FIGURE 18 For a cloverleaf with collector-distributor roads the loop travel time can be decreased for traffic moving from the major arterial to the freeway. " In summary, it can be stated that the traveltime for the left turn movement on a cloverleaf without collector-distributor roads is on the order of 60 seconds, if the left turning volume is less than the loop capacity. With the addition of a collector-distributor road this travel time is reduced to approximately 45: seconds for traffic moving from the major arterial to the freeway. The delay for left turning traffic moving from the freeway to the major arterial would be computed by the same procedure as utilized for the cloverleaf without collector~istributor roads. INTERCHANGE COST To facilitate a comparison of the cost between diamond and clover leaf interchanges it was necessary to develop a standard design situation and then apply a unit cost figure to each cost item for both types of interchanges. In order to include all of the cost items within the confines of the largest interchange. area required (outer loops, acceleration and deceleration lanes, and ramps) it was necessary to consider a 4800 foot section of roadway along the freeway and an 1800 foot section of road-. way for the arterial streets (see Figure 19) for this cost comparison ·study. To serve as a basis for a design, the volume condition shown in Figure 20 was assumed. The number of lanes and the interchange required· for both diamond and the cloverleaf were then developed. The estimated average daily traffic shown in Figure 20 was changed to design hourly volume (Figure 21) for both the A.M. and P.M. peaks.· Following this an actual flow volume was developed for each element of the diamond and the cloverleaf. The critical lane design procedure was used in the design of the diamond (Figure 22) and then evaluated to determine the number of lanes needed on the .four approaches for both the A.M. and P.M. peaks. The design of the cloverleaf was considered in two parts ( 1) the cloverleaf with collector-distributor roads (Figure 23) and (2) the cloverleaf without collector distributor roads (Figure 2 4). Also shown on Figures 23 and 24 are the actual flow volumes which were developed on the various elements of the cloverleaf for both the A.M. and P.M. peaks. 29 0 0 0 "'" 0 0 10 I(') 90 0~00 ~~.. ... 6000~.tJ c ...... ~-11000 16000 • ~ Hi/roo 0 0 0 "'"v ASSUMED VOLUME CONDITIONS COST COMPARISON STUDY FIGURE 19 After considering ~everal methods of determining interchange costs the decision was made to compare the cost of the diamond and the clover leaf on the following items: 1. Amount of earth work " 2. Square footage of bridge involved 3. Square yards of pavement involved 4. Acreages of right-of-way involved At the present time the exact per cent of the total cost that these four items Would represent is not known, but it is estimated that they constitute approximately 80 to 90 percent of the total cost of an inter change. Furthermore, it is believed that the other items such as guard rails, sodding grass, surfacing shoulders, lighting, etc. are minor items and that they will not make an appreciable difference in the total cost. It on this basis that this cost comparison is justified. Two types of cost estimates were developed. The first was a basic cost which constituted a total cost for all of the stated items within the confines of the largest right-of-way required as shown in Figure 25. The second was an overage cost or a cost to provide an interchange between the freeway and the at-grade arteriaL The basic assumption made in connection with the overage cost study was that for any interchange it would be necessary to provide a roadway for both the freeway and the at grade arterial. Therefore 1 the cost for design elements required for interchange (loops in case of cloverleafs and ramps for diamonds) could be estimated separately as overage cost. Tabulations of total interchange quantities and estimated basic overage cost for each item of the diamond and cloverleaf appear in Table A. It should be noted that in Table A the total basic pavement for the cloverleaf without collector-distributor roads 'is less than the diamond or the cloverleaf with collector-distributor roads. The quantity of pavement is less for the cloverleaf without collector distributor roads because for the frontage roads are discontinued at the outer loops as shown· in Figure 25. The volume of earthwork 1 the square footage of bridges, the square yards of pavement, and the acres of right-of-way were computed utilizing the maximum width of the typical freeway sections shown in Figure 26 i.e. a six lane divided freeway 1 three lane frontage roads 1 and four la~e arterial streets. A maximum of four per cent grade was used in the de sign of the diamond and the cloverleaf as shown in Figure 2 7. The end areas were computed using A+ H (w+3H) 1 a formula derived from the chart 31 g 0 'lit •• 11000 • • 16000• 0 0 0 ~ ~ .ASSUMED VOLUME CONDITIONS COST COMPARISON STUDY FIGURE 20 4000 3500 1750 A.M. PEAK 2275 2150 1750 3500 P.M. PEAK 4325 DESIGN HOURLY VOLUMES COST COMPARISON STUDY FIGURE 21 FREEWAY I I J 500 I I I I I L - - ' - - - -- .1500 lmttt · iEH l ....a 625. - - - -· -. - - 525• l I I I I I I A.M. PEAK FREEWAY J ~.~ I._____· ~I1~1 -----'1 L ..900 I I . I ff26•I I P.M. PEAK DIAMOND INTERCHANGE DESIGN COST COMPARISON STUDY FIGURE 22 500 A.M. PEAK 0 0 0 10 10 tO 625 ,._ 825 900 1025 1100 1500 P.M. PEAK CLOVERLEAF INTERCHANGE DESIGN· WITH COLLECTOR- DISTRIBUTOR ROADS COST COMPARISON STUDY FIGURE 23 1425 A.M. PEAK 0 !!? (IJ 825 P.M. PEAK 10,._ (IJ (IJ CLOVERLEAF INTERCHANGE WITHOUT COLLECTOR -DISTRIBUTOR ROADS COST COMPARISON STUDY FIGURE 24 4 LANE 3 LANE ARTERIAL FRONTAGE ROAD WIDTH=48' WIDTH = 36' - :> 1//c -- 6 LANE FREEWAY WIDTH= 72' > BASIC REQUIREMENTS FOR DIAMOND AND CLOVERLEAF WITH COLLECTOR - DISTRIBUTOR ROADS 3 LANE 6 LANE FREEWAY J WIDTH =72' FRONTAGE ROAD WIDTH =36' 4 LANEdff ARTERIAL WIDTH=48' - ~...:;..·;::::._-__=-+__, ....,.,.. 1!!f BASIC REQUIREMENTS FOR A CLOVERLEAF WITHOUT COLLECTOR - DISTRIBUTOR ROADS DESIGNS CONSIDERED FOR DETERMINING BASIC COSTS FIGURE 25 338' R.O.W. 15' 36 36' 15. TYPICAL SIX LANE FREEWAY SECTION TYPICAL RAMP SECTION 1"1:! USUAL~~·-----;ri't.~IN.)~::l--36' -t- J4,~---36' -L:i---(2~~.RiN.)~6' (USUAV ~~ ~~ . ----- 338' R.O.W. (MAX.)------~ TYPICAL SIX LANE FREEWAY SECTION WITH COLLECTOR- DISTRIBUTOR ROADS ~~~~~"~~~~~~~~"~ USE ON 6 LANE MAJOR STREET 1 1-4-----40 ------i"i 1---- 40'------.1...... ~, 6' 1-----24'-->l(36') t------:---_;_;..146-"--'-~_!I)~~E'-'L=EN~GT'-'-'-H ------1 ( 170' FOR 6 LANE MAJOR STREET) TYPICAL FREEWAY OVERPASS (TYPICAL MAJOR STREET SECTION UNDERPASS) DESIGN CONSIDERATIONS COST COMPARISON STUDY FIGURE 26 z (.!) CJ) lLl 0 4+50 IJJ (.!)z :; ::::> :; X 10 The unit price of each item included in this cost comparison was obtained by taking an average cost of each item from several interchanges that have been built. The unit process used was as follows: Earthwork- $1..30 per cubic yard Bridges - $7.00 per square foot Pavement~ $5.00 per square yard Right-of-way- $5,000 per acre It should be noted that the unit prices for both the basic and the overage . cost are the s arne with the exception of the pavements. A unit price of $6.00 per square yard was used for the overage cost on the pavement since. ramps would require additional grading and form work. The prices of each item listed above is an average cost obtained from past experience. Since these costs were used for both the diamond and the cloverleaf cost cal.:. culations the general cost comparisons would not be greatly affected if they are slightly in error. If the procedure outlined is utilized in future interchange cost studies, it is suggested that the best current price estimates available be utilized. · Since the price of right-of-way is a major cost item in the total cost of an interphange and since it is very difficult to obtain an average right-of-way cost that would apply in all situations, it was felt desirable to compute a family of right-of-way cost curves (Figure 29) which would show the relationship between the cost of the various types of interchanges, and the effect of right-of-way on the total cost for each type of interchange. Since the estimated unit price for earthwork, bridges, and pavement is essentially a standard figure f the effect of various costs of right-of-way on the total interchange cost is of particular interest. It should be noted that the right-of-way cost curves shown in Figure 29 were developed utilizing the estimated unit prices as outlined in this cost comparison study and that a similar set of curves should be developed if more current prices are utilized. 40 w ...J~ N LANES LANESM I~~ H L 3H .I 2 A =WH + 3 H =H ( W + 3H) 150 -t:i ~-- 100 0~~._~~~----~------~--~--~------~~ 0 1000 2000 3000 4000 5000 2 END AREA (FT ) END AREA COMPUTATIONS CHART COST -COMPARISON STUDY FIGURE 28 -CJ) a: <( ....J ..J 2.0 0 0 1.8 CJ) z 0 1.6 _J ...... J 1.4 -~ cnr- 1.0 0 0 .8 .L&J C> z .6 <( J: .4 0 a: LaJr .2 z o~~--~----~----~~--- o 2000 4000 6000 8000 10000 12000 14000 RIGHT-OF-WAY COST (DOLLARS PER ACRE) EFFECT OF RIGHT- OF-WAY COST ON INTERCHANGE COST FIGURE 29 ,.. .J.. ~. " -to::\ ~ ... SUMMARY AND ,CONCLUSIONS The research work on the project "Ramps and Interchanges" has included studies of entrance ramp operation, diamond interchange signalization and operation and cloverleaf operation. This report has presented data from com parative studies of diamond and cloverleaf interchanges which considered the factors of capacity I efficiency and cost. It is felt that the results of the previous studies, as listed in the intro duction plus the comparative data presented in this report, provide a factual guide to the selection and design of nondirectional interchanges. Since any interchange design problem presents individual problems related to specific volume and location factors, it is not possible to say that one interchange type is superior to another. However, it is possible with the data presented, to make factual comparisons of capacity efficiency and cost along the lines suggested in this report and to select the type interchange which will do the best job for the given design situation. 43 TABLE A ------TYPE OF INTERCHANGE CLOVERLEAF ITEM DIAMOND WITHOUT COLLECT6R- WITH COLLECTOR- DISTRIBUTOR ROADS DISTRIBUTOR ROADS QUANTITY COST QUANTITY COST QUANTITY COST EARTHWORK (BASIC) 19,198 CU. YD. $ 24,950.00 16,613 CU. YD. $. 2 1,600.00 19,198 CU. YD. $ 24,950~00 EARTHWORK (OVERAGE) 79,740 CU. YD. 103,662.00 97,146 CU.YD. 126,200.00 150;248 CU.YD. . 195,~00.00 BRIDGE 19,720 SQ.FT. 138,040.00 23,800 SQ.FT. 166,700.00 40,800 SQ.FT. 285,000.00 PAVEMENT (BASIC) e6,ooo SQ.FT. 430,000.00 72,000 SQ.FT. 360,000.00 86,000 SQ.FT. 430,000.00 PAVEMENT (OVERAGE) 32,000 SQ. FT. 19,200.00 19,917 SQ. FT. 119,700.00 15,267 SQ.FT. 91,602.00 RIGHT-OF-WAY (BASIC) 39.9 AC. 199,500.00 39.9 AC. 199,500.00 39.9AC. 199,500.00 RIGHT- OF -WAY (OVERAGE) 0.0 AC. 20.9 AC. 104,500.00 26.4AC. 132,000,00 TOTAL BASIC COST $ 654,450.00 $ 581,100.00 $ 654,450.00 TOTAL OVERAGE COST 260,902.00 517ll00.00 703;802.00 TOTAL COST $ 9«5, 352.00 $t,<*t,.!OO.OO $!,-1&2.00 . ------~ t ------~------~---·~'-----'--~ TOTAL QUANTITIES · COST COMPARISON STUDY -, .. (!:· .#>. .. "' ~~ ~- ~· ... v TABLE 8 TYPE OF INTERCHANGE AMOUNT AND TYPE OF DIAMOND CLOVERLEAF EARTHWORK WITHOUT WITH COLLECTOR-DISTRIBUTOR ROADS COLLECTOR- DISTRIBUTOR ROADS FILL FOR OVERPASS 79,740 CU. YD. 89,882 CU. YD. 143,300 CU. YD. FILL FOR LOOPS 7, 264 CU. YD. 6,947 CU. YD. AT-GRADE INTER- 19,198 CU. YD. 16,613 CU. YD. 19, 198 CU. YD. SECTION TOTAL FILL 98,938 CU. YD. 113,759 CU. YD. 169,446 CU. YD. TOTAL COST OF $128,619.00 $ 14 7,800.00 $220,150.00 EARTHWORK EARTHWORK QUANTITIES COST COMPARISON STUDY \.}