This story appears in the Aug. 25 print edition of Transport Topics.
A controversy over truck speed in Illinois has reignited debate over whether large differentials in truck and car maximum speed limits cause an increase in the number of crashes.
Illinois Gov. Pat Quinn earlier this month vetoed a bill pushed by the trucking association in that state that would have lowered the speed differential between cars and trucks. The speed limit for trucks would have gone up to 60 mph from 55 mph on rural interstates in several of the state’s counties.
By comparison, the top speed allowed for cars on the same highways is 70 mph — currently a difference of 15 mph.
Quinn said he vetoed the measure, which passed in the state’s House and Senate without a single dissenting vote, because increased speeds for trucks on interstate highways “will result in the increased loss of human life.”
In a letter to Illinois legislators he said: “Higher travel speeds lengthen stopping distances, making it more difficult to react to an emergency in time. Speed also exacerbates the size and weight differences between large trucks and passenger vehicles, leading to more severe crashes.”
Quinn cited four fatal accidents in the state to back up his veto.
But Matt Hart, executive director of the Illinois Trucking Association, said the crashes the governor cited were not relevant because the speeds in all four crash locations were the same for trucks and cars.
Although research is not definitive on the subject, Hart remains convinced that the current 15-mph speed differential is dangerous.
For that reason, Hart said, he sought and has received assurances from state legislative leaders that they will call for a vote to override Quinn’s veto as early as November.
Hart said his association did an analysis comparing rural interstates nationwide with uniform truck and car speed limits to Illinois, which has several interstates with split speed limits.
“It turned out that Illinois had a higher per-vehicle accident rate than all the other states that had the uniform 65 speed limits,” Hart said.
However, Russ Rader, senior vice president of communications for the Insurance Institute for Highway Safety, said the notion that variable speed limits between cars and trucks are unsafe is not supported by research.
“From a safety standpoint, we think it makes sense to have a lower speed limit for large trucks because of stopping distances,” Rader told Transport Topics. “Lower speed limits for trucks made the stopping distances closer to lighter vehicles, and lower truck speeds also allow passenger vehicles to pass trucks more easily.”
A yet-to-be released study for the Michigan Department of Transportation concluded that variable truck and car speed limits do not increase the number of fatal accidents, said Timothy Gates, a professor and co-investigator of the study done by Wayne State University’s Transportation Research Group in Detroit.
Maximum speed limits in Michigan are 70 mph for cars and 60 mph for trucks. Michigan is one of only eight states that have differential maximum speed limits, Gates said.
“There is really no discernable difference in the fatal crash rate if we just look at overall crash rates on rural interstates,” Gates said. Yet, there was some suggestion in the data that uniform speeds slightly elevated the number of fatal crashes, he added.
The bottom line: The study did not recommend that Michigan change the speed limits to be the same for trucks and cars, Gates said.
Daniel Murray, vice president of research for the American Transportation Research Institute, agreed that no study yet can conclude definitively that variable speed limits cause more crashes.
Still, Murray said research does suggest that, even when truck and car speed limits are the same, cars have a tendency to go faster than trucks, no matter what the posted speed limit is.
ATRI research from 2008 to 2010 showed that, when truck and car speed limits are both 75 mph, 92% of trucks complied with the speed limit. By contrast, only 49% of the cars complied, Murray said.
ATRI’s research also found that when the differential was only 5 mph or 10 mph, there was a tendency for crash exposure to increase significantly, Murray said.