Michigan to Study Split Truck-Car Speed Limits to Determine if There Are Safety Benefits

This story appears in the June 17 print edition of Transport Topics.

Researchers at Wayne State University in Detroit have been given a $175,000 grant to study whether different freeway speed limits for cars and trucks make roads safer or more dangerous.

Years of research on truck and car speed differentials have produced widely divergent conclusions, said Peter Savolainen, the Wayne State civil and environmental engineering professor who is principal researcher of the study being funded by the Michigan Department of Transportation.

Differential speed limits — currently in effect in eight states, including Michigan — became popular among state legislators in the 1960s based on the idea that trucks require greater time and distance to stop and are more difficult to handle than passenger cars.

“The thinking at that point was, to compensate for those facts, we’re going to limit how fast those trucks can drive,” Savolainen told Transport Topics.

Since then, one of the unintended consequences has been studies and anecdotal evidence suggesting that speed differentials by as much as 10 mph or 15 mph can lead to more crashes, Savolainen said.

So the study will attempt to further clear up the question of how differential speed limits affect safety, and it also will search for a “balanced optimal range” of speeds that will reduce crashes, Savolainen said.

The eight states that have a speed limit differential between cars and trucks are: Arkansas, California, Idaho, Indiana, Michigan, Montana, Oregon and Washington.

In recent years, Illinois, Ohio, Texas and Virginia have matched truck and car speed limits.

As a result, speed-limit variances are sweeping among states. For instance, it’s legal for a heavy truck to go 15 mph faster on some two-lane highways in Texas than on a rural interstate in California or Illinois, and there is a 20 mph difference in the speed limit on the same highway, Interstate 10, when a truck crosses the state line from California to Arizona.

Savolainen said the Wayne State study will include field research with radar guns to compare speeds on freeways in Indiana, Michigan and Ohio — states that share common roadways. It also will link up crash and fatality data with a number of factors ranging from truck traffic to inspection data.

“We’re really looking at a broad set of factors,” Savolainen said.

Indiana and Michigan have a 10 mph speed differential (higher for cars) while Ohio’s speed limits are the same for cars and trucks.

The Wayne State study’s results are to be reported to the Michigan Department of Transportation in May 2014.

However, American Trucking Associations maintains that speed variances can increase the risk for serious or fatal accidents.

Darrin Roth, ATA’s director of highway operations, said the federation is “generally opposed” to different speed limits for trucks and cars.

Any time a car or truck changes lanes, there is the potential for a crash, Roth said. From a common-sense and statistical perspective, that potential is even greater when a car and truck are traveling at different speeds, he added.

While it may seem logical that a car coming up fast on a slower-moving truck is potentially dangerous, prior speed-differential studies have reached different conclusions on how the variance actually has an effect on highway safety.

“Although there have been a number of studies that have investigated the safety implications of posted speed differentials between automobiles and heavy trucks, the results have been inconclusive,” said a 2009 University of Arkansas study.

In addition, some of the studies have been discredited because of political and technical issues, the Arkansas study noted.

Similarly, a 2005 Federal Highway Administration review of numerous studies of speed differentials noted that a distinction between differential and same speed limit safety factors “cannot be generalized but instead varies by site within a single state.”

Idaho, a state where cars since 1998 have been allowed to travel 10 mph faster than trucks, revisited the issue last year after a measure was introduced in the Legislature to increase the truck speed limit on rural interstates to 75 mph from 65 mph.

The Idaho Senate Transportation Committee asked the state Department of Transportation to assess whether the lower truck speed limit has had a positive effect on safety and, working with other agencies and stakeholders, to develop recommendations about whether the current policy should be continued.

The resulting study by the University of Idaho concluded that lowering the truck speed limit actually had a “noticeable safety benefit.”

“The policy favorably changed truck speeds, measured in terms of the reduction in average truck speed and the increase in the percentage of trucks complying with the posted speed limit,” the Idaho study said. “The state’s DSL [differential speed limit] policy also contributed to a reduction in the truck-related crash rate. Because of these observed safety improvements, we recommend that Idaho’s DSL policy be maintained to preserve current traffic safety trends.”