CHAPTER IX Traffic Operations of service that is intended for those tables. The tables are Introduction the highway. A level-of- not intended to show extreme service indicates traffic situations either in terms of conditions in terms of speed, roadway or vehicle Longer and heavier trucks freedom to maneuver, traffic characteristics; under tend to disrupt traffic flow on interruptions, comfort and different characteristics the roadways more than convenience, and safety. A PCEs could be higher than conventional vehicles. PCE represents the number of shown in those tables. However, more trucks of any passenger cars that would use size or weight would also the same amount of highway The PCEs for all the traffic disrupt traffic. Disruption capacity as the vehicle being on a given roadway increase occurs in the through traffic considered under the with increased sizes and lanes, at roadway prevailing roadway and weights of trucks and intersections, and on freeway traffic conditions. decrease with fewer trucks in interchanges. Common the traffic stream. The net measures of disruption Trucks are larger and, more effect of these opposing include hours of delay and importantly, accelerate more changes for each scenario congestion costs. slowly than passenger cars, analyzed is presented in this and thus have a greater effect chapter. This chapter presents on traffic flow than passenger estimates of changes in delay cars. On level terrain and in Table IX-1 shows PCEs for and associated congestion uncongested conditions trucks on rural highways. It costs resulting from the truck conventional trucks may be demonstrates that the highest size and weight (TS&W) equivalent to about two PCEs occurs on highways policies tested in the five passenger cars in terms of with the steepest grades and illustrative scenarios: their impact on traffic flow. highest speeds. Table IX-2 Uniformity, North American In hilly or mountainous terrain shows PCEs for trucks on Trade, Longer Combination and in congested traffic their urban highways. It again Vehicles (LCVs) Nationwide, effect on traffic flow often is shows the effect of highway H.R. 551, and Triples much greater and they may be speed on PCEs. After grade Nationwide. Qualitative equivalent to 15 or more and highway speed in assessments of other, related, passenger cars. The actual importance is the weight-to- impacts are also discussed. number of PCEs depends on horsepower ratio of the the operating speed and grade trucks. Basic Principles of the highway section, the vehicle’s length, and its Other Traffic Effects weight- to-horsepower ratio which is a measure of how the In addition to congestion, this Traffic Congestion vehicle can accelerate. Study has assessed, but not Tables IX-1 and IX-2 show quantified in detail, the Traffic congestion depends on PCEs for trucks operating in impact of longer and heavier the capacity of and the amount rural and urban areas under trucks on the operation of of traffic on a given highway. different conditions. The traffic in the areas of vehicle It is assessed in terms of effects of differences in truck offtracking, passing, passenger car equivalents length and weight-to- acceleration (including (PCE). Further, highway horsepower ratio is shown in merging, speed maintenance, capacity depends on the level Table IX-1. Vehicle Passenger Car Equivalents on Rural Highways Truck Length Grade Vehicle Weight-to- Horsepower Ratio (feet) Roadway Percen Length (pounds/horsepower Type 40 80 120 t (miles) ) 150 2.2 2.6 3.0 0 0.50 200 2.5 3.3 3.6 Four-Lane 250 3.1 3.4 4.0 Interstate 150 9.0 9.6 10.5 3 0.75 200 11.3 11.8 12.4 250 13.2 14.1 14.7 150 1.5 1.7 Not Simulated 0 0.50 200 1.7 1.8 Not Simulated Two-Lane 250 2.4 2.7 Not Simulated Highway 150 5.0 5.4 Not Simulated 4 0.75 200 8.2 8.9 Not Simulated 250 13.8 15.1 Not Simulated and hill climbing), lane speed. Dynamic high-speed changing, sight distance There are several measures of offtracking is a swinging back requirements, and clearance a vehicle's ability to negotiate and forth due to rapid steering times. As with congestion, turns or otherwise "fit" within inputs. On roadways with the speed (a function of the dimensions of the existing standard lane widths, the two weight, engine power, and highway system, but the high-speed offtracking effects roadway grade) and length of principal measure is low- are not large enough to be of a vehicle are the major factors speed offtracking. Two other concern. Excessive low- of concern, although vehicle measures are high-speed speed offtracking can disrupt speed is more important than offtracking and dynamic high- length in assessing congestion speed offtracking. High- effects. speed offtracking , is steady- state swing out of the rear of a combination vehicle going Offtracking through a gentle curve at high IX-2 Table IX-2. Vehicle Passenger Car Equivalents on Urban Highways Vehicle Weight-to- Truck Length Roadway Traffic Flow Horsepower Ratio Grade Type Condition (pounds/horsepower ) 40 80 120 150 2.0 2.5 2.5 Congested 0 200 2.5 3.0 3.0 250 3.0 3.0 3.0 Interstate 150 2.5 2.5 3.0 Uncongested 0 200 3.0 3.5 3.5 250 3.0 3.5 4.0 150 1.5 2.5 2.5 Congested 0 200 2.0 2.5 2.5 Freeway and 250 2.0 3.0 3.0 Expressway 150 2.0 2.0 2.0 Uncongested 0 200 2.5 2.5 2.5 250 3.0 3.0 3.0 150 2.0 2.0 2.5 Congested 0 200 2.0 2.0 3.0 Other 250 3.0 3.0 4.0 Principal Arterial 150 3.0 3.0 3.5 Uncongested 0 200 3.5 3.5 3.5 250 3.5 4.0 4.0 traffic operations and result in link between low-speed Chapter VII, Roadway shoulder or inside curb offtracking and the likelihood Geometry, for a detailed damage at intersections and at of serious crashes (fatal or discussion of offtracking. interchange ramp terminals injury-producing). This is designed like intersections due to the vehicle’s very low Standard STAA doubles (two that are used heavily by speed when turning sharply. 28-foot trailers) and triple- trucks. There is little, if any, The reader is referred to trailer combinations (three IX-3 Table IX-3. Effects of Speed Differentials on Crash which is of particular concern Involvement in cases where frequent truck- car conflicts can be anticipated. This issue needs to be addressed when Involvement Ratio Speed Differential Crash considering the ability of a (related to 0 speed (mph) Involvement given segment of roadway to differential) safely accommodate longer 0 247 1.00 and heavier trucks. Poor acceleration is a concern as it 5 481 1.95 can result in large speed 10 913 3.70 differentials between vehicles in traffic, and crash risks 15 2,193 8.88 increase significantly with 20 3,825 15.49 increasing speed differential. Table IX-3 indicates that 28-foot trailers) exhibit better crash involvement may be low-speed offtracking Operators of longer or from 15 times to 16 times performance than a standard heavier vehicles have to be more likely at a speed tractor and 48-foot or 53-foot more diligent to avoid differential of 20 miles-per- semitrailer combination, as potential passing conflicts. hour (mph). they have more articulation Standards for marking passing points in the vehicle and no-passing zones on two- As a vehicle's weight combination and use trailers lane roads, developed in the increases, its ability to with shorter wheelbases. 1930's, are based on cars accelerate quickly for passing cars. The operation merging with freeway traffic Passing or Being Passed on of trucks in these zones was and to maintain speed Two-Lane Roads not considered when these (especially when climbing standards were developed nor hills) is degraded, unless Cars passing LCVs on two- has it been considered since larger engines or different lane roads could need up to an then. However, this is gearing arrangements are 8 percent longer passing sight mitigated by the fact that truck used. These concerns may distance compared to passing drivers have a better view of also be addressed by existing tractor-semitrailer the road as they sit higher than screening routes to ensure combinations. For their part, car drivers. they are suitable for use by longer trucks would also any vehicle at its proposed require longer passing sight weight and dimensions. distances to safely pass cars Aerodynamic truck designs, on two-lane roads. Also Vehicle Acceleration by reducing drag, help trucks heavier trucks require more to accelerate and maintain engine power to pass another Acceleration performance speed as well. vehicle if it is necessary to determines a truck's basic accelerate to pass the ability to blend well with On routes with steep grades overtaken vehicle. other vehicles in traffic, IX-4 Table IX-4. Distribution of Grades on Arterial Highways Grade 6.50 or 0.00 - 0.49 0.50 - 2.49 2.50 - 4.49 4.50 - 6.49 (percent) more Miles of Highways 64.7 47.4 15.2 4.6 1.2 (thousands) Percent of 48.6 35.6 11.4 3.4 0.9 Total that are frequently traveled by The Highway Performance than 80 percent has a grade of trucks, special truck climbing Monitoring System (HPMS) no more than 2.5 percent. lanes have been built.