Comprehensive Truck Size and Weight Limits Study - Modal Shift Comparative Analysis Technical Report
Chapter 6: Scenario Impacts on Traffic Operations
Other performance measures such as density were considered, but the applications of these measures do not fit the scope of this project, which is aimed at evaluating truck impact on traffic operations in a national network.
This chapter presents estimates of changes in delay and associated congestion costs resulting from the truck size and weight policies tested in the six scenarios described in Chapter 2.
Traffic congestion is determined by the capacity of a given highway and the amount of traffic on it. In traffic engineering, the impact of trucks is assessed in terms of passenger car equivalents (PCE). A PCE represents the number of passenger cars that would use the same amount of highway capacity as the vehicle being considered under the prevailing roadway and traffic conditions. Further, highway capacity depends on the level of service that is intended for the highway. A level-of-service indicates traffic conditions in terms of speed, freedom to maneuver, traffic interruptions, comfort and convenience, and safety.
Trucks are larger and, more importantly, slower to accelerate to their desired speeds than passenger cars, which means they have a greater effect on traffic flow than individual passenger vehicles. The value of PCEs depends on the operating speed and grade of the highway section, the vehicle’s length, and its weight-to-horsepower ratio, which measures how a vehicle can accelerate. Previous research included in the 2000 CTSW Study indicated that on level terrain and in uncongested conditions conventional trucks may be equivalent to about two passenger cars in terms of their impact on traffic flow. In hilly or mountainous terrain and in heavily congested traffic, their effect on traffic flow often is much greater, and they may be equivalent to 15 or more passenger cars. Tables 30 and 31 show PCEs for trucks with different weight-to-horsepower ratios operating in rural and urban areas under different conditions. The effects of differences in truck length and weight-to-horsepower ratios are quantified in those tables.
These tables are taken directly from the 2000 CTSW Study, which is one of the few studies analyzing truck PCEs with different weight-to-horsepower ratios. PCEs shown in the tables were estimated using simulation modeling since actual observations of the impacts of the vehicle configurations being analyzed in that study were not readily available. Resource constraints for the 2000 CTSW Study precluded validation of the simulation results, but validation would have been difficult anyway because many of the vehicle configurations being analyzed did not operate widely across the country. The methods and findings of the traffic operations analysis for the 2000 CTSW Study were subject to peer review as were all other parts of that study. The desk scan conducted for this 2014 CTSW Study did not uncover any peer review of these values since the 2000 CTSW Study was conducted. It also did not find new research examining the relative PCEs of different vehicle configurations under different operating environments. There were several research studies since the 2000 CTSW Study that examined PCEs under different operating environments, such as at intersections; however, none of these studies were conducted with a large array of truck configurations that could be applied in this study. For example, the 2010 HCM provides average PCE values representing a fleet mix of trucks instead of unique PCEs for trucks with different weight-to-horsepower ratios. As such, these research findings were considered supplemental rather than a substitute to the 2000 CTSW Study.
While the state-of-the-art in simulation modeling has improved since the 2000 CTSW Study was conducted, there is no research suggesting these improvements would significantly affect the relative PCEs for the scenario and base case vehicles being analyzed in the current 2014 CTSW Study. This is particularly true since this operation analysis is a nationwide policy study that is not intended for highway planning and design purposes at the local level. For Scenarios 1-3, the absolute values of the PCEs for base case and scenario vehicles are not critical since under the assumption of this study weight-to-horsepower ratios would be maintained where possible, so the scenario and base case vehicles would have the same PCEs. Impacts on delay and congestion costs would be primarily a function of the relative VMT for the scenario and base case vehicles. For Scenarios 4-6, the improved simulation models available today compared to those that were available for the 2000 CTSW Study could give somewhat different PCE values for base case and scenario vehicles, but the relative difference in PCEs should not be significantly different.
Table 30 shows PCEs for trucks on rural highways. It demonstrates that the highest PCEs occur on highways with the steepest grades and highest speeds. Table 31 shows PCEs for trucks on urban highways. It again shows the effect of highway speed on PCEs. After grade and highway speed in importance is the weight-to- horsepower ratio of the trucks. Note that Tables 30 and 31 are not intended to show extreme situations either in terms of roadway or vehicle characteristics; under some different settings the PCEs could be higher than shown in these tables.
The PCEs for all the traffic on a given roadway increase with increased sizes and weights of trucks and decrease with fewer trucks in the traffic stream. The net effect of these opposing changes for each scenario analyzed is presented in this chapter.
Source: USDOT, 2000 Comprehensive Truck Size and Weight Study
Key: NS = Not Simulated.
Other Traffic Effects
In addition to congestion, this 2014 CTSW Study assesses, but does not quantify in detail, the impact of longer and heavier trucks on the operation of traffic in the areas of vehicle off-tracking, passing, acceleration (including merging, speed maintenance, and hill climbing), lane changing (including weaving), sight distance requirements, clearance times, pedestrian areas, and work zones. As with congestion, the speed (a function of weight, engine power, and roadway grade) and length of a vehicle are the major factors of concern, although vehicle speed is more important than length in assessing congestion effects.
There are several measures of a vehicle’s ability to negotiate turns or otherwise “fit” within the dimensions of the existing highway system, but the principal measure is low-speed off-tracking. Two other measures are high-speed off-tracking and dynamic high-speed off-tracking. High-speed off-tracking is a steady-state swing-out of the rear of a combination vehicle going through a gentle curve at high speed. Dynamic high-speed off-tracking is a swinging back and forth due to rapid steering inputs.
Source: USDOT, 2000 Comprehensive Truck Size and Weight Study
On roadways with standard lane widths, the two high-speed off-tracking effects are not large enough to be of concern. Excessive low-speed off-tracking, however, can disrupt traffic operations and result in shoulder or inside curb damage at intersections and interchange ramp terminals that are used heavily by trucks.
Passing or Being Passed on Two-Lane Roads
Cars passing LCVs on two-lane roads would need up to an 8 percent longer passing sight distance compared to passing existing tractor-semitrailer combinations. For their part, longer trucks would also require longer passing sight distances to safely pass cars on two-lane roads. Heavier trucks also require more engine power to pass another vehicle if it is necessary to accelerate to pass the overtaken vehicle.
Heavy truck operators must be particularly cautious when passing on two-lane roads because standards for marking passing and no-passing zones on two-lane roads, developed in the 1930s, are based on cars passing cars. The operation of trucks in these zones was not considered when these standards were developed, nor has it been considered since then. However, this is partially mitigated by the fact that truck drivers have a better view of the road as they sit higher than car drivers.
Vehicle Acceleration at Ramps and Steep Grades
Acceleration performance determines a truck's basic ability to blend well with other vehicles in traffic, particularly at ramps where the merging and diverging maneuvers are needed. As a vehicle's weight increases, its ability to accelerate quickly for merging with freeway traffic and to maintain speed (especially when climbing hills) is degraded, unless larger engines or different gearing arrangements are used. These concerns may also be addressed by local policies that identify specific routes to ensure they are suitable for use by any vehicle at its proposed weight and dimensions. Aerodynamic truck designs, by reducing drag, provide a promising vehicle design solution to help trucks to accelerate and maintain speed as well.
On routes with steep grades that are frequently traveled by trucks, special truck climbing lanes are often built to accommodate truck acceleration and deceleration capabilities in order to reduce the congestion and safety impacts to passenger vehicles. Otherwise, trucks are expected to maintain reasonable grade climbing performance. In the past, hill climbing performance has been addressed by requiring larger trucks to be equipped with higher horsepower engines. While in some cases larger engines may be necessary to maintain grade climbing performance, experience has shown that a more easily enforceable approach is to specify minimum acceptable speeds on grades and minimum acceptable times to accelerate from a stop to 50 mph or to accelerate from 30 mph to 50 mph.
The 2008 Highway Performance Monitoring System (HPMS) provided highway grade data for the 48 contiguous States and the District of Columbia. Table 32 summarizes the distribution of grades on different highway types.
Source: 2008 HPMS Data
In addition, highway design policies place limits on the steepness of grades. Federal policy for the Interstate System specifies maximum grades as a function of design speed. For example, highways with design speeds of 70 mph may not have grades exceeding 3 percent. Gradients may be up to 2 percent steeper than those limits in rugged terrain. Generally, the steepest grades to be encountered by heavy trucks are to be found in the mountainous areas of the western United States and, to a lesser extent, on some of the older highways in the northeastern States. As discussed previously, Table 30 shows the marked effect that percent and length of grades have on truck climbing ability if the truck does not have a low ratio of GVW to horsepower.
In previous studies, fleet owners who operate large trucks (mostly in the West) were asked about their experience with combination vehicles. They said they purchase trucks with large enough engines that allow drivers to maintain reasonable and efficient speeds. Tractor manufacturers corroborated this, indicating that trucking companies and individual drivers want and buy trucks with large engines. Engine manufacturers build engines with up to 600 horsepower. These engines are sufficient to maintain a minimum speed of 20 mph for a 130,000-pound truck on a 6 percent grade. Over the past 20 to 30 years, engine power has grown at a more rapid rate than weight.
If single-drive-axle tractors are used in multi-trailer combinations, the tractor may not be able to generate enough tractive effort to pull the combination up a hill under slippery road conditions, especially if it is heavily loaded. In these cases, either tandem- axle tractors or tractors equipped with automatic traction control would be appropriate. Specially built tractors are used in Colorado to push multi-trailer combinations when they have traction problems.
Lane Changing and Weaving
Compared to conventional tractor-semitrailer combinations, longer vehicles require larger gaps in traffic flow in order to change lanes or merge with traffic. The effect of this performance characteristic is proportional to vehicle length and the surrounding traffic density. Limited research is available to quantify the specific impact of truck lane changing. The HCM 2010 weaving analysis uses the same truck PCE values as basic freeway segments. Nevertheless, skilled truck drivers can minimize impacts to traffic by limiting the number of lane changes and using extra caution when merging and weaving.
Heavier vehicles entering traffic on two-lane roads from non-signalized intersections could take more time to accelerate up to the speed limit. If sight distances at the intersection are obstructed, approaching vehicles might have to decelerate abruptly, which could cause a crash or disrupt traffic flow. Longer trucks crossing non-signalized intersections from a stopped position on a minor road could increase by up to 10 percent the distance required for the driver of a car in the cross traffic to see the truck and bring the car to a stop without impacting the truck.
Table 33 shows how vehicle features affect traffic congestion, off-tracking, and operations. As indicated in the table, the most important parameters are vehicle length and weight, with speed being closely related to weight. Increases in allowable lengths may only be compensated for by limiting operations to multilane facilities except for short distances. Weight may be compensated for by requiring that vehicles be able to maintain sufficient speed in order to not disrupt traffic excessively on any route used.
Source: USDOT, 2000 Comprehensive Truck Size and Weight Study
+/- As parameter increases, the effect is positive or negative.
E = Relatively large effect. e = relatively small effect. — = no effect.
The impact of trucks on pedestrians and bicyclists is low and sporadic along most roadway segments. However, when the paths of two groups cross each other at intersections, there is a risk of severe injury to unprotected pedestrians/bicyclists. Increased truck length, off-tracking, and rearward amplification are factors that could adversely affect the safety of pedestrians and cyclists on highways with larger trucks. Better route planning, safety education, and better traffic control are common countermeasures to improve truck safety in the pedestrian areas.
Although work zones certainly present hazards to all drivers, trucks are affected in particular due to their larger size, heavier weight, and inability to accelerate and decelerate in the same distance as other traffic. Work zones can result in narrow lanes, speed-limit adjustments, lane shifts, and stop-and-go traffic. All of these can create difficult challenges for trucks. To the extent that increased truck size and weight limits might necessitate additional pavement and bridge repairs, the number of work zones and the resulting impacts on traffic operations could potentially increase. (For detailed information on these impacts, please see Volume II: Pavement Comparative Analysis and Volume II: Bridge Structure Comparative Analysis) In general, there may also be the potential for longer or heavier trucks to have different impacts in work zones.
Local traffic regulations and the development of transportation management plans (TMPs) for specific work zones can mitigate work zone truck impacts. FHWA requires the development of TMPs for all Federal-aid highway projects to reduce traffic and mobility impacts, improve safety, and promote coordination within and around the work zone. State and local transportation agencies need to develop and implement TMPs that best serve the mobility and safety needs of the local road users, construction workers, businesses, and community.
As noted above, the analysis of the impacts of the increased truck size and weight allowance scenarios on traffic operations drew heavily from data and methods developed for the 2000 CTSW Study. Highway user delay and congestion costs were assessed using PCE values developed in the 2000 CTSW Study. That study used three traffic simulation models—one for Interstate highways, one for rural two-lane highways, and one for urban arterials to estimate PCEs for trucks with different weights, dimensions, and performance characteristics in different highway environments. To obtain PCEs by truck length and gross weight-to- horsepower ratio, the simulation models were run many times for two sets of representative roadway geometric conditions for each of the three highway types. A detailed description of methods used to estimate PCEs for different truck characteristics in different highway environments is presented in the paper, "Quantifying Traffic Operational Impacts of New Truck Configurations in the U.S. Highway Network."
The truck vehicle miles of travel (VMT) by truck configuration and weight that is estimated to result from the increased truck size and weight allowance scenarios is substituted in the traffic delay model for the base case (2011) truck VMT, and the change in highway operating speed by functional class is calculated to obtain the change in delay for all highway users. This change in delay in vehicle hours is then multiplied by a time value of $17.24 per hour to obtain the change in congestion costs. This time value was taken from the Highway Economic Requirement System ($13.16 in 1994 dollars) and adjusted by the USDOT Guidance on Valuation of Travel Time in Economic Analysis that recommended a 1.6 percent compound growth rate for 2011.
Comparison to Previous TSW Studies
The 2014 CTSW Study is updated with several changes from the 2000 CTSW Study. These changes include updated speed-flow rate curves, updated roadway network based on the HPMS 2011 network data and an updated value of travel time that reflects the 2011 value.
The impacts of the policy scenarios on traffic – highway user delay, congestion costs, low-speed off-tracking, passing, acceleration (merging and hill climbing), lane changing, and intersection requirements – are discussed below.
Table 34 shows the changes in traffic delay and congestion costs for Scenarios 1 through 6. Overall the changes of VMT have minimal impact on travel speed. Nevertheless, all of the scenarios could marginally reduce delay and congestion costs compared to the base case.
Scenarios 3 and 4 could reduce delay and congestion costs over $850 million in 2011. This assumes that requirements are in place to ensure the heavier trucks have engines and braking systems with power sufficient to perform as existing trucks perform.
The remaining traffic operations impacts – off-tracking, passing, acceleration, lane changing, and intersection requirements – are evaluated in qualitative terms.
 FHWA, Work Zone Safety and Mobility Rules, https://ops.fhwa.dot.gov/wz/resources/final_rule.htm. Last modified September 19, 2013. Return to Footnote 9
 Elefteriadou, Lily and Nathan Webster, “Quantifying Traffic Operational Impacts of New Truck Configurations in the U.S. Highway Network,” 1997, http://road-transport-technology.org/Proceedings/5%20-%20ISHVWD/Part%201/QUANTIFYING%20TRAFFIC%20OPERATIONAL%20IMPACTS%20OF%20NEW%20TRUCK%20CONFIGURATIONS%20IN%20THE%20U.S.%20HIGHWAY%20NETWORK%20-%20Elefteriadou%20.pdf Return to Footnote 10
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