Review of U.S. Department of Transportation Truck Size and Weight Study - Second Report: Review of USDOT Technical Reports (2015) / Chapter Skim
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Pages 46-56
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From page 46... ...
44 6. SAFETY The MAP-21 study charge calls for two analyses of truck safety: a comparison of accident frequency and accident risk of vehicles operating under grandfather or other exemptions from federal size and weight limits [Section 32801(a)
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From page 47... ...
45 weight studies (e.g., USDOT 1981, II-22; TRB 1990a, 15; TRB 1990b, 129–130) included such quantitative comparisons of systemwide safety impacts of alternatives based on syntheses of information from analyses of crash data, results of research of others on crash rates, and vehicle stability simulation and testing.
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From page 48... ...
46 because "the method used to estimate configuration-specific VMT in the 2008 data differed from the 2011 data to the extent that the 2008 data were not usable as a second data point." This observation exposes the shortcoming of the WIM-derived VMT distribution estimates: the configuration distributions are sensitive to assumptions made in processing the WIM data. As one example of difficulties in interpreting WIM data, it was pointed out to the committee 10 that one axle on some Michigan six-axle tractor-semitrailers is a lift axle that is raised from the pavement when the truck is not loaded.
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From page 49... ...
47 the roads the vehicles use, and the distribution of the trucks' travel by time of day and season of the year. Therefore, in many circumstances, factors other than size and weight are likely to dominate safety comparisons among populations of vehicles.
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48 Michigan typically operate with the rearmost three axles widely spread, rather than in the tridem configuration illustrated in the report and assumed in the stability analysis (Safety, ES-3, 56)
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From page 51... ...
49 show a causal relationship; it may reflect differences in operating environments not considered in the analysis (e.g., a larger share of the alternative configuration's VMT may occur on congested roads, which tend to have higher crash rates but lower average crash severity than uncongested roads)
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From page 52... ...
50 AADT that may affect the risk comparisons. For example, any differences between truck types in patterns of use by time of day or in frequency of use of Interstate exits would likely influence relative crash rates.
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From page 53... ...
51 of truck stability similarly reported much worse rearward amplification for seven-axle triples than for five-axle doubles (Fancher and Mathew 1990, 127)
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From page 54... ...
52 Inspection and Violation Analysis The USDOT study compared the alternative configurations with the control vehicles with respect to rates of violation of safety rules found in roadside safety inspections conducted routinely by state enforcement officials. The analysis found that "tractor semitrailer configuration was not a significant predictor of the likelihood of a violation.
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From page 55... ...
53 Recommendations The USDOT study's effort to identify states with crash data and traffic data adequate to support estimates of crash rates for specific truck configurations was worthwhile. USDOT should continue to work with the states to develop data systems that can be used to monitor the safety performance of tractor-semitrailers operating within the federal weight limit, heavier tractor-semitrailers, and multitrailer combinations.
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From page 56... ...
54 for each road functional class, is unrealistic (TRB 1996, 69–72)
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