Trans-Talk

A study by University of Arkansas professor Steven Johnson examines interactions between automobiles and large trucks on rural, intersate highways. “Cost-Benefit Evaluation of Heavy Truck-Automobile Speed Differentials on Rural Interstate Highways,” compared speeds and accident rates of heavy trucks and automobiles in five states with speed-limit configurations ranging from a uniform 75 mph for cars and heavy trucks to a low differential of 65 mph for automobiles and 55 mph for trucks.

Johnson claims speed differentials lead to more vehicle interactions, and consequently more accidents, as faster vehicles pass slower ones. However, he also notes that higher accident rates resulting from passing incidents are offset by lower accident rates resulting from improved stopping abilities for trucks travelling at lower speeds.

“People argue that heavy trucks require longer braking distances for any given speed, and lower truck speeds help equalize the stopping distance,” Johnson said. “On the other hand, opponents of lower truck speed limits have suggested that the differential speeds increase speed variance and therefore have a negative impact on highway safety. Our research demonstrates that it is likely that both of these arguments are correct.”

Johnson and co-author Naveen Pawar also reported:

• Speed-limit enforcement influences variation in traffic speeds. If speed limits are not strictly enforced, motorists choose their own “comfortable” speed.
• Speed data collected in the study illustrated that, although posted speed limits for automobiles differed by 10 mph, average speeds differed by only 1.6 mph. In other words, regardless of whether the speed limit was set at 65 mph or 75 mph, motorists drove between 73.2 and 74.8 mph.
• If limits are set at what is considered to be arbitrarily low values, motorists will not adhere to those values. For example, compliance rates for the configuration of a uniform 70 mph were 31 percent for automobiles and 70 percent for trucks. By contrast, compliance rates for lower, posted differential limits — 65 mph for cars, 55 mph for trucks — were 7 percent for automobiles and 0 percent for trucks.
• Average truck speed is 3 to 4 mph slower than average auto speed, whether or not speed limits are uniform.
• Commercial trucking company policies that restrict the maximum speed of their fleet by using speed limiters — engine mechanisms that prevent trucks from traveling faster than a desired speed — increase the amount of speed variance on interstate highways (although fuel efficiency tends to be higher.)
• Speed variance, in addition to vehicle speed, significantly affects fuel efficiency and the amount of pollution as both trucks and automobiles must accelerate and decelerate to maneuver around slower traffic.

(Source: University of Arkansas Daily Headlines; Johnson’s report is available in HTML, PDF and MS Word format at http://www.mackblackwell.org/web/research/final-reports.htm.)

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