KALAMAZOO, Mich.—A team of three researchers from Western Michigan University has been awarded a $161,802 grant from the Michigan Department of Transportation to study whether changing speed limits on Michigan interstate freeways has had an impact on crash types and severity.
The grant was awarded to Drs. Valerian Kwigizile and Jun-Seok Oh, assistant professor and professor, respectively, of civil and construction engineering, and Dr. Hyunkeun Cho, assistant professor of statistics. The team will study how changes in speed limits have impacted the frequency and severity of crashes.
About the study
In 1996, Michigan passed legislation to increase the speed limit along certain segments of interstate highways from 65 to 70 mph, Kwigizile says. A study by Michigan State University shortly after that found the marginal increase in speed did result in an increase in accidents.
"Since it has been a long time, the Michigan Department of Transportation wants to conduct another study to see whether those findings are still valid," Kwigizile says. "And since we have more segments where speed limits have changed, they want to expand the study that was done by MSU and get better results."
The research team began gathering data in March and hopes to complete the study by December 2017. One would expect that raising the speed limit would result in more accidents, Kwigizile says. But that isn't necessarily so.
"Different studies have shown different results," Kwigizile says. "There are some studies that have shown that increasing speed does not increase the number of crashes. However, there is one finding that is consistent throughout studies, which is increasing speed increases the severity of crashes."
Sites selected across state
The team will study different freeways, but has not completed the site selection process. Sites will be spread throughout the state to capture different geographical characteristics. Researchers will look at crash data and speed profiles both before and after the speed limit was raised in 1996.
"We hope to see if there was an increase right after the speed limit was changed and then, after that, they went down," Kwigizile says.
The three researchers are being assisted in their project by two graduate students, but more will be added later, especially to collect field data and determine how fast cars are actually going. The team also will be sifting through data archived by MDOT.
The first objective will be to determine if the findings of the original MSU study in 1997 still hold true. The other objective is to see what the real impact is of changing speed limits.
Economic impact studied
"My own opinion is that it's going to have an impact on operational speed, it's probably going to have an impact on crashes, maybe not on the number of crashes but on the severity of those accidents, and it has an economic impact," Kwigizile says. "That economic impact is coming from the fact that if you are raising the speed limit you have to modify the infrastructure to accommodate the increased speed. If you have a roadway curve, you have to adjust the way that curve is designed."
Another economic impact results from the reduced travel time with higher speeds, Kwigizile adds.
"Since time has a value, instead of spending two hours on the road, maybe the users are going to spend one-and-a-half hours," Kwigizile says. "The half-hour that they saved is money."
The team will do an economic analysis to assess the cost of increasing speed limits, including the cost of crashes and the cost of infrastructure changes, but also the benefits of shorter travel time. Other aspects of economic analysis will include emissions, noise impacts, pavement impacts and fuel consumption
"The state is in interested in knowing if what they are doing is cost beneficial," Kwigizile says.
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