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Researchers Study Impacts of Differential Speed Limits

Interstate 94 at the Michigan Indiana line
Michigan is one of eight states that post differing speed limits for trucks and passenger cars. Now civil engineers at Wayne State University, along with other experts, are examining what effect differential speeds have on safety. Wikimedia Commons/Dan Perry

Examining previous studies, vehicle speeds, and historical data, researchers seek insight into the role that different speed limits for different types of vehicles may play in road safety.

July 2, 2013—Fifty years ago, a majority of states had speed limits that were different for passenger vehicles than for semitrucks and buses. But now only eight states have differential speed limits, and some of those are considering whether the practice should continue. To shed light on the issue, researchers in one of those states are studying the impacts of differential speed limits on road safety, travel speeds, and other factors.

The Michigan Department of Transportation has awarded a $175,000 contract to researchers at Wayne State University, who along with other experts will examine that state’s differential speed limit, which is 70 mph for passenger vehicles and 60 mph for trucks and buses on rural freeways. On roads on which the speed limit for passenger vehicles is less than 70 mph, the limit for trucks and buses is 55 mph. Other states with differential speed limits are Indiana, Arkansas, California, Idaho, Montana, Oregon, and Washington.

For decades it was assumed that lower speed limits for trucks and buses would improve safety because these larger vehicles require more time to stop and are more difficult to maneuver on curves. But previous research has suggested that differential speed limits result in variable travel speeds that can actually increase the potential for crashes. “Consequently, the purpose of this study is to provide decision makers with a policy analysis of what the impacts would be of maintaining or raising the truck/bus speed limit,” said Peter Savolainen, Ph.D., P.E., M.ASCE, an associate professor of civil and environmental engineering at Wayne State University and a principal researcher on the project, who wrote in response to questions posed by Civil Engineering online.

Researchers will review earlier speed limit studies and collect historical data from Michigan and other states—particularly those that have transitioned from differential speed limits to uniform speed limits or vice versa—to determine how trends in traffic crashes and fatalities vary between them. “One of the great advantages of doing transportation research currently is that data quality is continually improving, and much more data are becoming available to us,” Savolainen said. “This is important because truck-involved crashes and fatalities, for example, are influenced by a wide range of factors, including traffic volumes, weather conditions, roadway geometry, and, of course, speed limits. By controlling for these other factors, we are able to better isolate the specific impacts of speed limits.”

Working with students, researchers will go into the field with radar guns to perform speed studies of vehicles traveling on the state’s freeways. Michigan is particularly well-suited to the research because several of its freeways extend into neighboring Indiana and Ohio, which are on different ends of the speed-limit spectrum. “Indiana has a five-mile-per-hour differential limit, so its trucks have a limit that’s five miles below passenger vehicles; we have a ten-mile-per-hour differential limit; and Ohio has no differential limit,” Savolainen noted. “So we can do a case-control study to see how speeds change as you go across the border to where the speed limits change.”

Although the posted speed limit may call for a differential, it’s probably safe to assume that most vehicles travel at a rate that exceeds those limits. Therefore, if Michigan decides to increase its truck and bus speed limit by 10 miles per hour, it’s unlikely that those vehicles will actually travel 10 miles per hour faster, Savolainen said. “My expectation would be that the actual increase in speeds would be less than ten, maybe three to six miles per hour,” he said. “But, in any case, it probably wouldn’t be the full ten miles per hour just due to the fact that trucks and buses are already going above the speed limit.”

Researchers will also consider factors that have not been examined as part of previous differential speed limit studies, including how increasing the speed limit may impact the trucking industry. “Intuitively, most expect that these companies would want the limits raised as this would allow them to deliver goods in a more timely manner,” Savolainen said. “However, these vehicles become much less fuel-efficient at high speeds. To that end, a number of companies install speed-limiting devices, which prohibit their trucks from going more than a threshold value, such as sixty-five or sixty-eight miles per hour.”

Other considerations will include whether pavement damage would increase as a result of heavy vehicles traveling at faster rates—a particular concern given that Michigan has some of the highest gross vehicle weight limits in the nation—and how raising the speed limit may impact air quality due to increases in emissions. For this portion of the project, researchers will examine prior studies in consultation with William Schneider, Ph.D., P.E., an assistant professor of civil engineering the University of Akron and a member of the research team who has expertise in air quality. “The primary focus of our analysis, at least in terms of the data we will be collecting, will be related to safety and speed,” Savolainen said. “Our assessment of impacts on emissions and pavement condition will principally involve a synthesis of prior research.” The research team also includes national experts from Purdue University and Wade Trim, a consulting engineering firm based in Detroit, who have conducted important research in this area.

The study is scheduled to be completed on May 1, 2014. Researchers believe the results could have implications nationwide, particularly for the seven other states with differential speed limits. The research could also provide insight for states that are considering increasing speed limits for all vehicles. “There are a few other states that are looking at higher speeds,” Savolainen said. “To my knowledge Michigan hasn’t talked about raising the overall speed limit. We’re just trying to examine this issue of whether it’s best to have trucks at the same speed or at a lower speed.”



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