Citation

Garrott WR, Heitz M, Bean B. Accid. Reconstr. J. 2012; 22(3): 45-57.

Copyright

(Copyright © 2012, Accident Reconstruction Journal)

DOI

unavailable

PMID

unavailable

Abstract

On July 27, 2009, the National Highway Traffic Administration (NHTSA) published a final rule reducing the maximum allowable truck-tractor stopping distances from 60 mph. FMVSS No. 121 also contains maximum allowable stopping distances for vehicles that cannot attain a speed of 60 mph in two miles. In this case, a vehicle is tested from an initial speed four to eight mph less than the maximum attainable speed. NHTSA received a petition for reconsideration of the July 27, 2009 final rule requesting that the agency revise the table of stopping distances for those truck tractors that cannot attain an initial test speed of 60 mph. The objective of this research was to obtain data on the stopping performance of one truck tractor‐semitrailer combination vehicle from a range of initial speeds. The truck tractor tested was a 1991 Volvo 6x4 tractor towing a 28 foot long, unbraked control trailer. A decision was made to modify the loading from the normal FMVSS No. 121 loaded condition. The loading was changed such that the 60 mph stopping distance specified in FMVSS No. 121 was just achieved (the Modified Gross Vehicle Weight Rating (MGVWR) loading). This vehicle was also tested at its Lightly Loaded Vehicle Weight (LLVW). For MGVWR loading, as initial speed is decreased from 60 mph, average measured corrected stopping distance drops faster than maximum permitted stopping distance until an initial speed of 35 mph is reached. The MGVWR vehicle had its largest margin of compliance at 35 mph. As initial speed decreased from 35 mph, maximum permitted stopping distance drops faster than average measured corrected stopping distance. The MGVWR vehicle had a negative margin of compliance at an initial speed of 20 mph. The maximum permitted stopping distance for the LLVW vehicle is greater than average measured corrected stopping distance for all initial speeds. The LLVW vehicle had its smallest (though still positive) margin of compliance at an initial speed of 20 mph. Average MGVWR and LLVW deceleration rise times, 0.43 and 0.30 seconds, respectively, are both less than the 0.45 second rise time that was used to calculate the maximum permitted stopping distances in FMVSS No. 121. Actual measured steady state deceleration is greater than FMVSS No. 121 assumed deceleration for the MGVWR loading condition for all initial speeds. This is surprising since actual measured corrected stopping distance exceeds maximum permitted stopping distance at an initial speed of 20 mph. This indicates possible discrepancies between the idealized deceleration shape used to calculate the FMVSS No. 121 values and the actual deceleration shape. The actual measured steady state deceleration is substantially greater than the FMVSS No. 121 assumed deceleration for the LLVW loading condition for all initial speeds. The equation FMVSS No. 121 uses to calculate maximum permitted stopping distances for initial speed below 60 mph was used to calculate steady state vehicle decelerations. The measured steady state decelerations are consistently higher than the steady state decelerations calculated using the FMVSS No. 121 equation. The magnitude of the difference increases with decreasing initial speed. The reasons for the difference between the two steady state decelerations are not known. It is also unknown which of these methods provides the best estimate for steady state deceleration. In terms of the overall goal of this research, it probably does not matter which estimate of steady state deceleration is best. At an initial speed of 20 mph, measured average corrected stopping distance for the MGVWR loading exceeded the maximum stopping distance permitted by FMVSS No. 121.