Safety in Numbers

| May 29, 2007

Eleven states have split truck/car speed limits on rural interstates, most with a 10 mph differential.
- Insurance Institute for Highway Safety

Thanks to state laws and carriers’ use of speed governors, the motoring public has been living with split car/truck speed limits for years. Whether this creates a problem continues to be debated.

Split speeds create safety hazards, says Todd Spencer, executive vice president of the Owner-Operator Independent Drivers Association. Take Ohio, where rural Interstates have 65/55 mph limits.

“All the trucks are congregated in the right-hand lane,” Spencer says. “It makes it harder for people to get on and off the highway.” Consequently, four-wheelers begin speeding or making inappropriate lane changes to work around the solid lanes of trucks. “You actually condition people to drive stupid,” Spencer says.

A 2006 University of Arkansas study comes down largely against speed differentials. “Data from previous studies and simple logic say that a higher number of interactions among vehicles increases the chance that accidents will occur,” says Steven Johnson, a UA industrial engineering professor, though he acknowledges that “lower truck speeds help equalize the stopping distance” between heavy trucks and four-wheelers.

Cirillo’s testimony noted that the controversial relationship between speed limits and safety has spawned many studies. “There has been no evidence to alter Solomon’s original finding that variance from the mean operating speed is a major contributor to accidents,” she said. “Many safety organizations and states, including Ohio, advise drivers to ‘drive with the flow of traffic.’”

Trucks, however, already average 6 mph slower than four-wheelers in highway traffic, Blower says, “so I don’t think there’s any strong evidence in the literature that there would be a negative safety impact” if trucks were limited to 68 mph. That level of governing would probably change the types of crashes – for example, more cars rear-ending trucks – but the bottom line could be fewer severe crashes, he says.

Road Safe America’s Owings disputes the differential speed argument for the same reason, only he estimates the current Interstate differentials are far higher than 6 mph.

Both speed and speed differentials are important factors in accidents, says IIHS, but “the risk of death and severe injury is a direct exponential function of speed, not speed differences.” Also, “Institute research found that lower speed limits for trucks on 65 mph highways reduced the proportion traveling faster than 70 mph without increasing variation among vehicle speeds. The percentage of trucks traveling faster than 70 mph was twice as large in states with uniform 65 mph limits.”

“The solution is not to let trucks go faster and faster to keep up with the passenger vehicles,” says IIHS’s McCartt. “The solution is to make all speeds safer.”

“In 2004, 86 percent of speeding-related fatalities occurred on roads that were not Interstate highways.”
- NHTSA report

Thirty-two states have different speed limits for rural and urban Interstates.
- Insurance Institute for Highway Safety

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