Study Shows Speed Limit Differentials Compromise Highway Safety

FAYETTEVILLE, Ark. — A new transportation study by University of Arkansas researchers reveals that different speed limits for cars and large trucks on rural, interstate highways lead to greater speed variation and a higher number of vehicles passing each other, thus compromising safety.

“We found that speed variation and vehicle interactions have a direct impact on highway safety,” said Steven Johnson, UA professor of industrial engineering with the Mack Blackwell National Rural Transportation Study Center. “Data from previous studies and simple logic say that a higher number of interactions among vehicles increases the chances that accidents will occur. Speed differentials -- 75 miles per hour for regular automobiles and 65 for large trucks, for example -- are a result of state-mandated speed limits and company policies that limit most trucks to a maximum speed of 62 to 65 miles per hour.”

Johnson reported this finding in “Cost-Benefit Evaluation of Heavy Truck-Automobile Speed Differentials on Rural Interstate Highways,” a comprehensive study of speed limits and car-vs.-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 an extensive review and analysis of existing literature on speed limits, Johnson and Naveen Pawar, a graduate student in the College of Engineering, measured driver behavior -- in other words, speeds -- 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. The researchers also collected and analyzed speed, accident and maintenance data and conducted hundreds of interviews with various stakeholders, including truck drivers; safety and maintenance managers of commercial trucking companies; and original equipment manufacturers of trucks, tires and engines.

There is a caveat, however, to the primary finding that differentials have a negative impact on safety: Johnson and Pawar agree with proponents of lower truck speed limits that 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.”

If transportation researchers, trucking-company safety personnel and motorists agree that highways are safer when vehicles travel at or close to a uniform speed, why do so many states have different speed limits for large trucks and cars? Furthermore, why is there so much speed-limit inconsistency from state to state? The argument that braking distances for slower-moving trucks is one reason and a perception of better fuel efficiency is another, but many factors unrelated to safety, road conditions and traffic influence decisions on setting speed limits.

“The large number of safety studies indicates that this issue has received a great amount of attention,” Johnson said. “Unfortunately, many of these studies involve more advocacy than science. It’s not that these studies aren’t valuable, but they have problems with methodology, statistical analysis and even simple understanding of important terms, such as 'speeding.’ In accident data, speeding is defined as both 'traveling faster than the posted limit’ and 'traveling too fast for conditions.’ Studies often do not differentiate between these definitions.”

Johnson and Nawar reported these additional findings:

• Speed-limit enforcement influences variation in traffic speeds. If speed limits are not strictly enforced, motorists choose their own “comfortable” speed.

• Motorists tend to drive at speeds at which they are comfortable, regardless of posted speed limits. For example, 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. Johnson found significantly different compliance rates for the various speed-limit configurations. 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.

• Despite many motorists’ perception that tractor-trailers pass them frequently, Johnson found that the average speed of trucks is 3 to 4 mph slower than the average speed of automobiles, even when speed limits were uniform.

• Changes in posted speed limits affect speed variances. After a speed-limit change, there is a transition period and an adaptation period. During the transition period, some drivers adapt slowly to the higher limits while others immediately travel at or above the new limit. The behavioral difference of these two groups increases the amount of speed variance and results in a temporary but artificial indication of higher accident rates due to increased limits.

• 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.

• Driving time has a significant impact on truck-driver fatigue. However, up to the average traffic speed, higher speed does not result in additional fatigue.

• 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.