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Comprehensive Truck Size and Weight Limits Study - Highway Safety and Truck Crash Comparative Analysis Technical Report

Chapter 5: Scenario Analysis

The goal of this analysis was to estimate for each scenario the annual changes in the number of rural and urban Interstate crashes that would be expected to occur if the alternative truck configurations were to operate nationwide on Interstates and National Highway System (NHS) roadways. As previously discussed, vehicle weight-based data was constrained primarily to Interstate System roadways, rendering an analysis of NHS roadways impossible.

In each scenario, the number of annual crashes was to be calculated for a “base” condition in which the nationwide Interstate VMT is based on the current operation of the alternative configuration (i.e., operation in a limited number of States) and for a “scenario” condition in which nationwide Interstate VMT is based on allowing the alternative trucks to operate in all States. The base and scenario Interstate VMT estimates that were to be used here are those described in Chapter 3 of the Volume II: Modal Shift Comparative Analysis and are the same estimates used in other areas of the study.

The change in crashes between the base and scenario conditions in each scenario was to be calculated as follows:

  1. Calculate base condition crashes for each truck configuration – For each truck configuration affected by the scenario (i.e., five-axle semitrailer (control single) and six-axle semitrailer configurations in Scenario 2 and Scenario 3; five-axle semitrailer (control double) and seven- and nine-axle triple configurations in Scenario 5 and Scenario 6), multiply the base condition VMT for that truck configuration by the crash rate for that configuration. The crash rates that were be used are those calculated from the State crash data as described earlier and in Chapter 2 of this report.
  2. Sum base condition crashes for each configuration to develop the total number of base-condition crashes.
  3. Calculate scenario condition crashes (assumes operation throughout the United States) for each truck configuration – For each truck configuration affected, multiply the scenario condition VMT for that truck configuration by the crash rate for that configuration. The crash rates that were to be used are again those calculated from the State crash data as described earlier.
  4. Sum scenario condition crashes for each configuration to develop the total number of scenario condition crashes.
  5. Calculate the change in total crashes by subtracting base condition crashes (Step 2 output) from scenario condition crashes (Step 4 output).
  6. Calculate the percent change in crashes by dividing the change (Step 5 output) by the base condition total (Step 2 output).

Calculations were initially attempted for Scenarios 2, 3, 5 and 6. In each Scenario, the calculations were planned to be conducted separately for urban Interstates and rural Interstates and then would be summed to produce results for Total (urban plus rural) Interstates. As noted before, analyses could not be conducted for Scenarios 1 and Scenario 4 since crash rates could not be developed for the alternative truck configurations in these scenarios.

The crash rates that were to be used in these calculations are from one State (Washington) for Scenario 2 and two States (Idaho and Michigan) for Scenario 3. The team believes these are the best estimates that can be currently obtained for the crash implications of the scenarios to be investigated. Ultimately due to the limited number of States with suitable data, FHWA determined the analysis of crash rates cannot generally be extended to other States or be used to forecast national impacts.

Both Scenarios 2 and 3 include the alternative truck configuration of a six-axle tractor semitrailer. Scenario 2 has a GVW of 91,000 lb., while Scenario 3 has a 97,000 lb. GVW. The control vehicle in both scenarios is a five-axle tractor semitrailer with a GVW of 80,000 lb. The crash rates used for Scenario 2 are based on the analysis of Washington State data, where the allowable GVW for six-axle tractor semitrailers is approximately 92,000 lb., which is very close to the 91,000 lb. alternative configuration target value. The crash rates used for the Scenario 3 analysis are a combination of the rates from Michigan and Idaho. The allowable GVW in both States is 105,500 lb., which is higher than the 97,000 lb. alternative configuration target.

The concept underlying the development of the estimates of changes in truck crashes within each scenario required two components – nationally-representative crash rates for each truck configuration and estimates of national VMT for both a base case of existing truck configurations and roadway networks and for a Scenario case involving alternative configurations and roadway networks. The study team faced significant data limitation including:

  • The findings for the Scenario 2 91,000 lb. 3-S3 configuration and the findings for the Scenario 5 and 6 triple-trailer configurations were each based on crash rates from one State. The findings for the Scenario 3 97,000 lb. 3-S3 configuration were based on crash rates from two States. The use of rates from this limited number of States clearly raises questions concerning whether these rates can be considered nationally representative, and thus concerning whether using them to predict nationwide estimates is appropriate.
  • Analysis was only attempted on Interstate roadways. It was not possible to even attempt to conduct analyses on non-Interstate roads due to limitations in both the crash and exposure data.
  • Since there is no information on operating GVW for each truck in State crash data, the definition of truck crashes used in the different scenarios was based on trailer and axle counts and GVW limits in the States. Whether or not the actual GVWs for trucks in the fleet analyzed in this study will be similar to the actual GVWs of an expanded fleet in the future is unknown.
  • The composition of the future fleets of alternative vehicles may differ from the current fleet that was analyzed in other unknown ways. For example, the same alternative configuration analyzed here (e.g., 129,000 lb. triple-trailer configurations) may carry different commodities in the future. If so, the carriers may differ, which in turn may cause the “safety culture” to differ (e.g., driver training, driver experience, truck maintenance procedures, equipment age, etc.) The effect of such possible differences could not be analyzed here. For example, while crash data contains information on driver age, there is no driver-age-specific truck exposure data, a critical need in any analysis of driver age effects.

FHWA believed these data limitations raise significant questions concerning the accuracy, reliability and validity of any nationally-representative crash rate estimates that could be calculated for each truck configuration. Thus, meaningful national level results could not be developed for this study.

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