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Comprehensive Truck Size & Weight Study

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TS&W Final Report

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Impact Analysis Areas

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Summary: Changes in the Nation's TS&W limits, which determine the maximum payload that vehicles may carry, will influence motor carrier productivity. For high-density--weigh-out--freight such as farm products and natural resources, a vehicle's maximum payload is controlled by truck weight limits. For low-density--cube-out--freight, such as computer equipment and snack foods, vehicle size limits constrain payload as opposed to weight limits.

In general, increases in TS&W limits would increase the tonnage and/or volume of freight that may be carried per vehicle per trip. Consequently, fewer trips would be required to carry the same amount of freight, decreasing vehicle-miles-of-travel (VMT) and reducing trucking costs. Alternatively, more restrictive TS&W limits would increase trips, VMT, and trucking costs.

Changes in truck costs and rates may cause, for some shipments, a change in the selection of transport mode. For example, changes in truck rates could induce some shippers to switch from rail to truck services. Further, changes in other shipper logistics costs impacted by TS&W variables (such as the size and frequency of shipments) may also influence intermodal (truck/rail) diversion. Examples of these costs include warehousing, order processing, and loss and damage.

A collection of state-of-the-art diversion models was developed for the CTS&W Study to predict the impact of TS&W changes on mode choice and truck configuration selection. For long-haul shipments, the Intermodal Transportation and Inventory Costs (ITIC) Model estimates each shipment's transportation and logistics costs for rail, rail-intermodal, and eight truck configurations for a large database of truck and rail shipments. The lowest cost mode is selected.

Short-haul truck shipments, defined as those under 200 miles, are analyzed separately from the ITIC model. Rail-to-truck diversion is not an issue for this market segment because trucks operating under 200 miles typically do not compete directly with rail traffic. Short haul diversion is estimated separately for heavy single unit and combination trucks.

For straight trucks, a procedure was developed to estimate: (1) the change in truck weight distributions within a truck configuration class, and (2) the change from one vehicle configuration to another that would likely result from changes in TS&W limits. It assumes that carriers will consider purchase price, operating costs and likely productivity improvements and will adopt the most economical equipment options into their fleets. For combination trucks, diversion estimates are extrapolated from the long-haul diversion model.

The determination of inter- and intra-modal diversion is extremely important to the overall CTS&W Study as most of the impact analysis methodologies require input regarding VMT by mode and truck configuration type.

Summary: Beyond the issue of motor carrier productivity is that of shipper costs. The motor carrier industry is considered sufficiently competitive that cost savings are assumed to be passed on to shippers as lower rates. This is generally true of the rail industry as well. A shipper that can shift to more productive truck configurations would realize lower total transportation and logistic costs. However, rail shippers that could not economically switch to trucks might face increased costs as railroads spread fixed costs over a smaller shipper base. Inter- and intra-modal diversion, therefore, has the potential to change costs borne by the Nation's shippers.

The ITIC model captures the impact of reduced truck costs for shippers using motor carrier services and for those rail customers which experience lower rates resulting from rail industry attempts to maintain traffic in the face of lower truck rates. However, the impact of freight diversion from rail to truck on the rates for the remaining rail customers and the viability of the rail industry is addressed using an independent analysis.

Specifically, the rail analysis estimates the necessary increase in rates for traffic remaining on the rail system after diversion. These increases would result from the fact that less traffic would be available to cover fixed costs. The contribution to capital lost from diverted traffic would be re-couped by increasing rates for the remaining traffic, potentially impacting future demand for rail service and therefore the financial status of the rail industry.

Summary: Considerable debate has focused on the safety of larger and heavier trucks, and whether allowing TS&W limits to increase would affect safety. Safety is a primary issue in the CTS&W Study because of the great public concern about the implications of mixing large trucks with passenger cars on our highways and because of the Department's enhanced priority on safety as the pre-eminent Departmental goal.

As discussed earlier in the section on freight diversion, providing the opportunity to deploy vehicles of differing sizes, weights, and/or configurations will alter travel patterns, changing the distribution of VMT by configuration and roadway type. As a result, the overall motor carrier accident experience may change, both in terms of accident rates and aggregate numbers. Quantifying the new safety profile on an aggregated basis, however, is extraordinarily difficult because historical accident rates can not be reliably applied to the new travel patterns as they would reflect what would have occurred under existing operating conditions and not what could occur under new conditions.

New truck travel patterns would likely result if the scenario vehicles were allowed to operate in significantly different environments than in the past. More specifically, certain configurations could operate in different regions of the country, on different functional classes, under different weather conditions and at different times of the day relative to the current fleet of vehicles.

Another factor complicating the estimation of accident rates given changes to TS&W policies is the fact that the population of commercial trucks represents a small subgroup of all vehicles, and consequently, there is a shortage of data directly correlating TS&W factors to type, frequency, and cause of roadway crashes.

Further, TS&W effects must be isolated from other safety variables before precise numbers of accidents may be determined. The physical characteristics of vehicles play a role in motor carrier safety experiences along with the important and interrelated factors of driver performance, roadway design and the traffic environment.

Although accident rates may not be reliably predicted for each scenario, valuable information about relative vehicle stability and control properties is available. Work commissioned for the CTS&W Study focused on comparing the dynamic properties of scenario vehicles with the vehicles they would be replacing.

Vehicle performance tests and engineering analyses indicate significant differences in the stability and control properties of different sizes, weights, and configurations of trucks. Some larger and heavier trucks are more prone to experiencing a rollover event than are other trucks; some are less capable of successfully avoiding an unforeseen obstacle when traveling at highway speeds; some negotiate tight turns and exit ramps better than others; some can be more reliably stopped in shorter distances than can others; and some climb hills and maneuver in traffic better than others.

Differing vehicle stability and control properties combined with new truck travel patterns will affect accident rates and numbers. For example, all vehicles (including trucks) traveling on lower standard roads experience significantly increased crash risk compared to those traveling on Interstate and other higher quality roadways. The majority of fatal crashes involving trucks occur on highways with lower standards. Also, higher traffic densities in populous areas exacerbate this problem.

In addition to safety impacts, the introduction of new truck configurations could have significant effects on the operational characteristics and quality of service on the highway network. The Study examines passenger car equivalents (PCEs) for a variety of truck configurations and provides estimates of the differences in overall delay that may occur with operation of the new truck configurations. The associated cost of this delay is also included in the Study.

Summary: Pavement wear is of interest because rough pavement affects the cost of travel. These costs include vehicle operating costs, delay, and crash or accident costs. The life of a pavement is determined by a number of factors: vehicle loading (axle loads, tire pressure and GVW), traffic volume and mix, environment, subgrade condition, initial pavement design, initial construction practices, maintenance and pavement age.

According to engineering principles, pavement deterioration increases with axle weight and with the number of axle loadings which a pavement experiences. The Study relies on FHWA's NAPCOM model to simulate pavement deterioration patterns given alternative vehicle weights, and axle configurations (with axle-group weight limits generally not changed) and to predict the requirement for road maintenance and construction expenditures. The NAPCOM model was developed to estimate cost responsibilities for the Department's 1997 Highway Cost Allocation (HCA) Study.

Summary: While the relationship between pavement deterioration and axle or axle group weight is well documented, the role of trucks with respect to bridge wear is not as well understood. Bridge engineers base new bridge designs on expected truck loadings with a safety margin to ensure against failure. These margins are significant and reflect uncertainty about bridge materials, construction practices and actual loads.

There is much controversy regarding the margin of safety appropriate for loads crossing a bridge. Most bridges in the United States were designed based on one of two standard loadings. "HS-20" designs are typical for Interstates and other highways where heavy truck traffic is expected. "H-15" loadings were used to design older bridges which are still in use and generally appear on the lower order functional class facilities. It should be noted that when bridges on lower order systems are replaced, they typically are replaced by HS-20 designs. Changes in TS&W limits may impact these safety margins, possibly increasing the number of bridges that must be replaced or posted.

The Federal bridge formula generally establishes truck axle spacings and loadings to limit bridge stresses to no more than 30 percent above the design loading of an H-15 bridge and no more than 5 percent above the design loading of an HS-20 bridge. As noted, design loadings incorporate significant margins of safety such that even the 30 percent overstress allowed on H-15 bridges does not put that bridge in danger of sudden failure.

The bridge formula reflects the fact that loads concentrated over a short distance are generally more damaging to bridges than loads spread over a longer distance. It provides for additional gross weight as the wheel base lengthens and the number of axles increases. It should be noted that the number of axles influences pavement wear more than bridge stress.

State transportation agencies rate bridges using an "inventory rating" or an "operating rating" approach to determine when a bridge should be posted to prevent its use by certain vehicles. The inventory rating is more conservative than the operating rating, requiring a greater margin of safety (55 percent of yield stress as opposed to 75 percent of yield stress for the operating rating). Past TS&W studies used either the inventory, operating or some compromise assumption between the two, to indicate the requirement for bridge replacement or posting. Not surprisingly, studies using the inventory rating found more bridges needing replacement and therefore, higher bridge replacement costs than studies using the other assumptions.

The current Study uses the bridge stress criteria as established by Bridge Formula B to indicate bridge replacement requirements; Bridge Formula B is not used. This means that for H-15 bridges, 71.5 percent of yield indicates the requirement for bridge replacement; for HS-20 bridges, 57.8 percent of yield indicates the need for replacement.

The Study relies on a state-of-the-art bridge stress engineering model to predict the impact of changes in truck traffic on bridge conditions. The model generates bridge reconstruction and maintenance capital costs as well as user delay costs resulting from bridge construction activities.

Summary:In some cases, the scenario vehicles will perform differently than vehicles in the current fleet. For example, long double-trailer combinations may have difficulty negotiating interchange ramps. In addition, some require staging areas where they can be assembled or broken down, allowing pickup and delivery with shorter combinations. Such performance characteristics may necessitate modifications to existing roadway geometric design features.

Work commissioned for this Study examined the relationship between the operating characteristics of the replacement configurations and the geometric elements of the current system. Geometric improvements required to accommodate the new vehicle operational characteristics were determined. The cost of upgrading roadway geometry as well as the cost of providing staging areas are estimated.

Data from nine States were examined and facilities with geometric designs that were problematic from the standpoint of current or scenario vehicles were identified. For analysis purposes, this sample was then expanded to cover all the States. The underlying assumption was that all interchanges would be reconstructed to meet the requirements of the "worst" vehicles. Of particular interest is the issue of low-speed offtracking. This phenomenon refers to slow turns where the rear wheels of a turning vehicle do not follow the same path as its front wheels.

Summary: Environmental impacts being evaluated for the CTS&W Study include air and noise pollution. Procedures developed for the HCA Study are being applied for the CTS&W Study. In general, environmental quality and energy consumption impact assessments are a function of VMT.

Motor vehicles produce emissions that damage the quality of the environment and adversely affect the health of human and animal populations. The cost of changes in air pollution levels resulting from alternative TS&W policy scenarios are not currently available. The Department is working with Environmental Protection Agency to develop estimates that adequately reflect the latest understanding of the costs of motor vehicle emissions. If this work is complete by the July Conference, the approach will be presented.

Noise emissions from motor vehicle traffic are a major source of annoyance, particularly in residential areas. Vehicle weight is a key variable affecting noise emissions. Noise costs were estimated using information on the reduction in residential property values caused by noise emissions. Estimates of noise emissions and noise levels at specified distances from the roadway were developed using FHWA noise models.

Also of interest are greenhouse gas emissions. Most scientists believe that increasing concentrations of greenhouse gases in the atmosphere will cause climate changes. Because of the tremendous uncertainty in climate change costs, no estimates of cost related to highway transportation are developed for this Study. However, changes in greenhouse gas emissions associated with the TS&W scenarios would be directly related to changes in VMT.

The change in fuel consumption given alternative vehicle configurations is also of interest. This was estimated, for each scenario, based on fuel economy by vehicle weight using engine performance models. Fuel consumption is also a function of VMT.

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