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. provided the highway grade Otherwise, trucks should be data for the 48 contiguous In addition, highway design able to maintain reasonable States and the District of policies place limits on the grade climbing performance. Columbia. The highway types steepness of grades. Federal In the past, hill climbing examined were rural freeway, policy for the Interstate performance has been rural multilane, rural two- System specifies maximum addressed by requiring larger lane, urban freeway, and grades as a function of design trucks to be equipped with urban arterial. Table IX-4 speed. For example, higher horsepower engines. summarizes this information highways with design speeds However, this type of by mileage. It shows that of 70 mph may not have specification can be almost half of the highway grades exceeding 3 percent. counterproductive, since system has a grade of no more Gradients may be up to 2 larger engines consume more than 0.5 percent and that more percent steeper than those fuel and emit more air pollutants. While in some cases, larger engines may be Figure IX-1. Highway Performance Monitoring System necessary to maintain grade climbing performance, The Highway Performance Monitoring System database is the experience has shown that a primary source of information for the Federal government more easily enforced about the Nation’s highway infrastructure. This is the most approach is to specify comprehensive nationwide data system in use for any aspect minimum acceptable speeds of the Nation’s infrastructure. Data collection is the on grades and minimum responsibility of the States, and it is updated each year. The acceptable times to States forward the data to the Federal Highway accelerate from a stop to 50 Administration, which maintains and uses these data for a mph or to accelerate from 30 variety of strategic planning and highway investment mph to 50 mph. evaluation uses. The Office of Highway Policy Information is responsible for receiving, reviewing, and tabulating these Grades data.

IX-5 limits in rugged terrain. and accelerate better than they Generally, the steepest grades ever have. Heavier vehicles entering to be encountered by heavy traffic on two-lane roads from trucks are to be found in the Traction unsignalized intersections mountainous areas of the could take more time to western United States, and to If single-drive-axle tractors accelerate up to the speed a lesser extent, on some of the are used in multitrailer limit. If sight distances at the older highways in the combinations, the tractor may intersection are obstructed, northeastern States. not be able to generate enough approaching vehicles might tractive effort to pull the have to decelerate abruptly, Table IX-1 shows the marked combination up a hill under which could cause a crash or effect that percent and length slippery road conditions, disrupt traffic flow. Longer of grade have on truck especially if it is heavily trucks crossing unsignalized climbing ability if the truck loaded. In these cases, either intersections from a stopped does not have a low ratio of tandem- axle tractors or position on a minor road GVW to horsepower. tractors equipped with could increase by up to 10 automatic traction control percent the distance required Industry Experience with would be appropriate. for the driver of a car in the Heavier Trucks Specially built tractors are cross traffic to see the truck used in Colorado to push and bring the car to a stop Fleet owners who operate multitrailer combinations without impacting the truck. large trucks (mostly in the when they have traction West), were asked about their problems. How truck size (dimensions), experience with combination design features, loading vehicles. They said they Lane Changing (weight distribution), and purchase trucks with large operation affect traffic enough engines that allow Compared to conventional congestion, offtracking, drivers to maintain reasonable tractor-semitrailer passing, acceleration, lane and efficient speeds. Tractor combinations, longer vehicles manufacturers corroborated require larger gaps in traffic this, indicating that trucking flows in order to change lanes companies and individual or merge with traffic. Skilled drivers want and buy trucks drivers can compensate for with large engines. Engine this vehicle property by manufacturers build engines minimizing the number of with up to 600 horsepower. lane changes they make and These engines are sufficient to using extra caution when maintain a minimum speed of merging. The effect of this 20 mph for a 130,000-pound performance characteristic is truck on a 6 percent grade. proportional to vehicle length and the traffic densities in Over the past 20 to 30 years, which a given vehicle engine power has grown at a operates. more rapid rate than weight. Trucks today maintain speed Intersection Requirements

IX-6 Table IX-5. Traffic Operations Impacts of Truck Size and Weight Limits

Vehicle Traffic Operations Offtracking Traffic Vehicle Features Acceleration Intersection Congestion Low High Lane Passing (merging and Require- Speed Speed Changing hill climbing) ments

Length - e - E + e - E — - E - E

Size Width — - e + e - e — - e —

Height — — - e — — — —

Number of units — + E - E — — - e — Design Type of hitching — + e + E — — + E —

Number of Axles — + e + e — — + e —

Gross vehicle - e — - E - E - E - e - E weight Loading Center of gravity — — - e — — - e — height

Speed + E + E - E - E — + e + E Operation Steering — - E - E — — - E — input +/- As parameter increases, the effect is positive or negative. E = Relatively large effect. e = relatively small effect. -- = no effect.

changing, and intersection maintain sufficient speed in more axles to be allowed to requirements are shown in order to not disrupt traffic carry more weight. Table IX-5 This table shows excessively on any route used. that the important parameters are vehicle length and weight A feature of each scenario with speed closely related to that eliminates certain traffic weight. Increases in impacts is that axle loads are allowable lengths may only be not increased. This means compensated for by limiting that there is no increased operations to multilane demand on any one set of facilities except for short brakes for stopping or distances. Weight may be descending long steep grades compensated for by requiring due to trucks being heavier as, that vehicles be able to necessarily, they must have

IX-7 increased fuel consumption As reference numbers for the Analytical Approach and inflation for 1994. delay and congestion cost for each scenario, the estimated delay on U.S. highways in Highway user delay and Assessment of Scenario 1994 is 18.7 billion hours and congestion costs were Impacts the costs for this aggregate assessed using three traffic delay were estimated to have simulation models—one for been $246.5 billion. This Interstate highways, one for The impacts of the policy estimate is based on data in rural two-lane highways, and scenarios on traffic -- Highway Information one for urban arterials. As highway user delay, Quarterly, June 1998, Office these models are sensitive to congestion costs, low-speed of Highway Policy vehicle length, gross weight, offtracking, passing, Information, FHWA and VMT and engine power, the acceleration (merging and hill estimates from the DOT’s analysis for this Study is climbing), lane changing, 1997 Federal Highway Cost sensitive to these factors. To intersection requirements -- Allocation Study. With no obtain PCEs by truck length are discussed below. change in TS&W policy, in and gross weight-to- the year 2000 the aggregate horsepower ratio, the models It can be seen that the Triples delay and associated costs were run for two sets of Nationwide scenario, which are estimated to increase by representative roadway would increase the weight 19 percent to 22.3 billion geometric conditions for each limit significantly, could hours and $292.9 billion of the three highway types. reduce delay and congestion respectively. costs by up to 7.6 percent in The truck vehicle-miles-of- 2000. This assumes that Vehicle offtracking is travel (VMT) by truck requirements are in place to assessed in terms of the costs configuration and weight that ensure the heavier trucks have to improve geometric features is estimated to result from engines with power sufficient to the extent necessary to new TS&W policy scenarios to perform as existing trucks remove any traffic operations is substituted in the traffic perform. Truck engines with problem that results from delay model for the base case enough power to accelerate a excessive offtracking. These truck VMT, and the change in truck up to traffic speed and costs are included in Chapter highway operating speed by to maintain speed on grades at VII, Roadway Geometry, and functional class is calculated the same performance level as discussed here in qualitative to obtain the change in delay 80,000-pound vehicles are terms. The remaining traffic for all highway users. This available on the market today operations impacts-- passing, change in delay in vehicle for combinations weighing up acceleration, lane changing, hours is then multiplied by a to 130,000 pounds. and intersection requirements time value of $13.16 per hour Regarding time to pass or -- are also discussed in to obtain the change in clear intersections, the longest qualitative terms. congestion costs. This value truck combinations would was taken from the Highway require from 10 percent to 15 Economic Requirement percent more time for these System ($10.92 in 1990 traffic maneuvers than a five- dollars) and adjusted for axle semitrailer combination.

IX-8 Table IX-6. Uniformity Scenario Traffic Impacts

2000 2000 Impact 1994 (base case) (scenario) Traffic Delay 18,700 22,300 22,400 (million vehicle-hours)

Congestion Costs 246,500 292,900 294,800 ($million)

Improvement for roadways Some degradation from 1994 on which long doubles now Low-Speed Offtracking resulting from VMT increase operate but would not in the for long double combinations future.

Some degradation from 1994 Negligible change over 2000 Passing resulting from VMT increase base case

Acceleration Some degradation from 1994 Negligible change over 2000 (merging and hill resulting from VMT increase base case climbing)

Some degradation from 1994 Negligible change over 2000 Lane Changing resulting from VMT increase base case

Intersection Some degradation from 1994 Negligible change over 2000 Requirements resulting from VMT increase base case

Uniformity Scenario in a small way across all Scenario impacts (see Table IX-7). As a result of the shift of However, for some of the The large increase in LCV freight from heavier and impacts, this is based on the use resulting from this longer vehicles to five-axle assumption the requirements scenario would have several semitrailer combinations at are in place to ensure that adverse effects if their 80,000 pounds, this scenario increased engine power for operations were not restricted would increase traffic those configurations with (see Table IX-8). congestion and associated increased gross vehicle costs in the year 2000 by 0.4 weights. Traffic delay and The scenario assumes these percent (see Table IX-6). congestion costs would be traffic operations problems slightly more (0.2 percent) in would be addressed by North American Trade 2000 than they would be restricting the use of these Scenarios otherwise. LCVs to multilane divided

These scenarios are estimated Longer Combination to improve traffic operations Vehicles Nationwide

IX-9 Table IX-7. North American Trade Scenarios Traffic Impacts

2000 2000 Impact 1994 (base case) (scenario) Traffic Delay 18,700 22,300 22,000 (million vehicle-hours)

Congestion Costs 246,500 292,900 289,500 ($million)

Some degradation from 1994 No impact. Featured vehicle Low-Speed Offtracking resulting from VMT increase off-tracks the same or less for long double combinations than baseline vehicle

Some degradation from 1994 Requires operating Passing resulting from VMT increase restrictions.

Acceleration Some degradation from 1994 Requires sufficient engine (merging and hill resulting from VMT increase power. climbing)

Some degradation due to additional length. Some degradation from 1994 Lane Changing (This is counterbalanced by resulting from VMT increase large decrease in heavy truck VMT.)

Some degradation due to additional length. Intersection Some degradation from 1994 (This is counterbalanced by Requirements resulting from VMT increase large decrease in heavy truck VMT.)

highways with entry and exit traffic flow through freeway This scenario, by eliminating only at interchanges where interchanges and up steep semitrailers longer than 53 needed improvements have grades. However, it is feet, will somewhat improve been made. Otherwise, traffic estimated that this scenario traffic flow through operations and safety could be would reduce user delay and intersections where these expected to be degraded on congestion costs by 3 percent longer trailers now operate. two-lane highways and during below that which can Beyond this, as shown in periods of peak traffic otherwise be expected in Table IX-9, its impacts are congestion. As these LCVs 2000. negligible. are heavier, as well as longer, provision for adequate engine power would need to be H.R. 551 Scenario required to ensure smooth

IX-10 Table IX-8. Longer Combinations Nationwide Scenario Traffic Impacts

2000 2000 Impact 1994 (base case) (scenario) Traffic Delay 18,700 22,300 21,600 (million vehicle-hours)

Congestion Costs 246,500 292,900 284,300 ($million)

Significant degradation Some degradation from 1994 (27.0 feet for turnpike Low-Speed Offtracking resulting from VMT increase double versus 16.5 feet for for long double combinations semitrailer)

Some degradation from 1994 Requires operating Passing resulting from VMT increase restrictions.

Acceleration Some degradation from 1994 Requires sufficient engine (merging and hill resulting from VMT increase power. climbing)

Some degradation due to additional length. Some degradation from 1994 Lane Changing (This is counterbalanced by resulting from VMT increase large decrease in heavy truck VMT.)

Requires operating restrictions (LCVs should not operate Intersection Some degradation from 1994 through intersections with Requirements resulting from VMT increase significant traffic volumes or insufficient sight distances for other traffic.)

Triples Nationwide Scenario and additional weight remain significant concerns in regard As with the LCVs to traffic operations. Also, Nationwide Scenario, this this scenario can be expected scenario would result in a to reduce highway user delay large increase in the use of and congestion cost by 8 triple-trailer combinations. percent from that which can However, offtracking is not a be expected in 2000 (see problem for triple-trailer Table IX-10). combinations, although length

IX-11 Table IX-9. Triples Nationwide Scenario Traffic Impacts

2000 2000 Impact 1994 (base case) (scenario) Traffic Delay 18,700 22,300 20,600 (million vehicle-hours)

Congestion Costs 246,500 292,900 270,500 ($million)

Some improvement as a Some degradation from 1994 triple trailer combination Low-Speed Offtracking resulting from VMT increase offtracks less (12.7 versus for long double combinations 16.5 feet) than semitrailer combinations.

Some degradation from 1994 Requires operating Passing resulting from VMT increase restrictions.

Acceleration Some degradation from 1994 Requires sufficient engine (merging and hill resulting from VMT increase power. climbing)

Some degradation due to Some degradation from 1994 additional length which is Lane Changing resulting from VMT increase counterbalanced by decrease in heavy truck VMT.

Additional length requires Intersection Some degradation from 1994 sufficient sight distances for Requirements resulting from VMT increase other traffic.

IX-12 Table IX-10. Triples Nationwide Scenario Traffic Impacts

2000 2000 Impact 1994 (base case) (scenario) Traffic Delay 18,700 22,300 20,600 (million vehicle-hours)

Congestion Costs 246,500 292,900 270,500 ($million)

Some improvement as a Some degradation from 1994 triple trailer combination Low-Speed Offtracking resulting from VMT increase offtracks less (12.7 versus for long double combinations 16.5 feet) than semitrailer combinations.

Some degradation from 1994 Requires operating Passing resulting from VMT increase restrictions.

Acceleration Some degradation from 1994 Requires sufficient engine (merging and hill resulting from VMT increase power. climbing)

Some degradation due to Some degradation from 1994 additional length which is Lane Changing resulting from VMT increase counterbalanced by decrease in heavy truck VMT.

Additional length requires Intersection Some degradation from 1994 sufficient sight distances for Requirements resulting from VMT increase other traffic.

IX-13