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Speed Safety
A transportation study by engineering researchers reveals that different speed limits for cars and large trucks on rural, interstate highways can compromise safety.
“We found that speed variation and vehicle interactions have a direct impact on highway safety,” said Steven Johnson, professor of industrial engineering with the Mack Blackwell Transportation Center. “Data from previous studies and simple logic say that a higher number of interactions among vehicles increases the chances that accidents will occur.”
Johnson reported this finding in Cost-Benefit Evaluation of Heavy Truck-Automobile Speed Differentials on Rural Interstate Highways, a study of speed limits and car-versus-large-truck speed differentials on rural, interstate highways. Sponsored by the U.S. Department of Transportation, commercial trucking companies and private fleets, the study will help regulatory agencies and trucking-company decision-makers establish policies regarding speed limits and differentials for both heavy trucks and automobiles.
After examining existing literature on speed limits, Johnson and graduate student Naveen Pawar measured the speeds of heavy trucks and automobiles in five states. Speed-limit configurations ranged from a uniform 75 mph for cars and heavy trucks to 65 mph for automobiles and 55 mph for trucks. The researchers also analyzed speed, accident and maintenance data and interviewed truck drivers, safety and maintenance managers of commercial trucking companies and original equipment manufacturers of trucks, tires and engines.
There is a caveat to their findings: vehicle dynamics, such as braking and maneuvering, improve on slower-moving trucks.
“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.”
See Johnson’s report at www.mackblackwell.org/web/research/final-reports.htm.