The case for speed limiters: 'More advocacy than science'?

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"It is evident that there is a need for additional research in many of the areas relevant to the maximum speed for heavy trucks. The decisions pertaining to the state regulated absolute and/or differential speed limits for trucks will continue to be a political, as well as a technical issue.” —Mack-Blackwell Rural Transportation Center study “Cost-Benefit Evaluation of Large Truck-Automobile Speed Limits Differentials on Rural Interstate Highways”

Is the Dept. of Transportation “cherry picking” research to support the rulemaking to require speed limiters in heavy-duty commercial vehicles? At least one source—a study that is cited often as providing evidence to support the rule—raises a number of concerns the DOT glosses over, or ignores altogether, including speed differentials, driver fatigue, fuel efficiency, and the overall cost-benefit analysis.

The first footnote in the “executive summary” of the DOT proposal refers to a 2005 paper, “Cost-Benefit Evaluation of Large Truck-Automobile Speed Limits Differentials on Rural Interstate Highways,” by researchers at the Mack-Blackwell Rural Transportation Center (MBRTC), a College of Engineering program at the University of Arkansas.

In that initial cite, and in three other footnotes, the paper “confirmed the common-sense conclusion that the severity of a crash increases with increased travel speed”—as, indeed, basic physics determines.

However, while the DOT rulemaking mentions the issue of speed differentials, the 118-page document does not reference the MBRTC report with regard to its primary subject matter. Indeed, after opening the “Safety Benefits” section by citing MBRTC, the proposal refers to two other studies that “observed no consistent safety effects of differential speed limits compared to uniform speed limits.”

The MBRTC report, a survey of available research at the time, also cites those “inconclusive studies,” and says the research did not address the impact of voluntary speed limiters, prevalent even 10 years ago, and so those studies were “inherently flawed.”

More broadly, “the large number of safety studies that were discussed in the literature review indicates that this issue has received a great amount of attention,” the researchers write. “Unfortunately, many of the studies involve more advocacy than science.”

The MBRTC study notes that proponents of lower truck speed limits argue that trucks require longer braking distances for any given speed and lower truck speeds help equalize the stopping distance. Truck drivers surveyed by the researchers, however, contend that their higher seat position allows a longer sight distance (multiple vehicles forward), reducing the effect of the differences in braking distance (to say nothing of the greater stopping power of modern disc-brakes and other safety technologies). The truck drivers are more concerned with the negative effect of greater speed variation and the number of interactions among vehicles.

“It is likely that both of these arguments are correct,” the paper says. “This would indicate that differential speed limits have two effects:

  1. the positive effect that results from improved vehicle dynamics (braking and maneuvering) for trucks at lower speeds; and
  2. the negative effect of increasing speed variation and the number of interactions among vehicles.

"These two effects of differential speed limits act in opposite directions and ultimately result in no observable effect on highway safety data.”

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