How is traffic safety affected by changes in traffic speeds following speed limit increases? An evaluation with probe vehicle data




75 mph, crash frequency modelling, rural freeway, speed limit increase, speed variance


Maximum speed limits continue to be an important policy issue. Research has consistently shown speeds to increase following speed limit increases. These increases have also generally coincided with increases in both the frequency and severity of crashes. However, research has been more limited as to the direct relationship between speed and safety. To this end, the present study evaluates the effects of increasing speed limits on the safety of rural limited access freeways while accounting for contemporaneous changes in the speed profiles of these same roadways. The speed limit increases in Michigan occurred between May 1 and June 12 of 2017, when the maximum speed limits were increased from 70 to 75 mph on approximately 600 miles of freeways. The maximum speed limits for trucks were also increased from 60 to 65 mph on all freeways state-wide at this same time. Speed data were obtained for the Michigan rural freeway network through probe vehicles. These data are merged with pertinent roadway data, as well as police-reported crash data at various levels of injury severity. The impacts of the speed limit increase on safety were evaluated by estimating random effects negative binomial models as part of a case-control before-after study design. The results show that the locations where the speed limits were increased experienced a 5% increase in crashes, while a marginal reduction in crashes was observed where speed limits did not increase. Interestingly, mean speeds were found to be negatively associated with crash frequency, while the standard deviation of speed was found to exhibit a positive relationship. Several site-specific characteristics were also found to be strong predictors of crash frequency. The results provide important insights into the nature of the relationship between speed and safety and will help to guide subsequent speed limit policy decisions.


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Author Biographies

Nischal Gupta, Michigan State University, USA

Nischal Gupta is a third year PhD student and graduate research assistant in the Department of Civil and Environmental Engineering. His research interests mainly focus on traffic safety and operations. He is currently working on multiple research projects including investigating the operational and safety benefits of changes in speed limits, effect of COVID-19 on traffic patterns, and supporting Ford Motor Company in the continued development and refinement of Ford’s Safety Insights.

CRediT statement: Conceptualization, Data curationFormal analysis, Investigation, Methodology, Writing—original draft.

Megat Usamah Megat Johari, Michigan State University, USA

Megat Usamah Megat Johari is a research associate at Michigan State University in the Department of Civil and Environmental Engineering. His research interest includes transportation safety, highway design, traffic operations, and driver behavior. He is involved in various projects including, updating the policy on geometric design of highways and streets; evaluating the effectiveness of safety messages on dynamic message sign; assessing the impact of speed limit increase on traffic safety and operations; and developing a comprehensive access management policy and guidelines.

CRediT statement: Formal analysis, Methodology, Visualization, Writing—original draft.

Hisham Jashami, Oregon State University, USA

Dr. Hisham Jashami is an Assistant Professor (Sr Res) of Transportation Engineering in the School of Civil and Construction Engineering at Oregon State University (OSU) and a certified Road Safety Professional Engineer. Prior to his appointment at OSU, Hisham worked as a Research Associate in the Department of Civil and Environmental Engineering at Michigan State University (MSU). His research has been focused on topics related to transportation safety, human factors, driving & bicycling simulators, autonomous vehicle simulation, and data analysis. During the course of his academic career, Dr. Jashami has worked on externally funded research from agencies including the National Highway Traffic Safety Administration, the Federal Highway Administration, the Pacific Northwest Transportation Consortium (PacTrans), and various State Departments of Transportation which has led to the publication of more than 35 peer-reviewed journal articles, conference papers, and technical reports.

CRediT statement: Formal analysis, Validation, Visualization, Writing—review & editing.

Peter T. Savolainen, Michigan State University, USA

Dr. Peter Savolainen is an MSU Foundation Professor and Associate Chair for Graduate Studies in the Department of Civil and Environmental Engineering. His research expertise covers a diverse range of projects in traffic safety, human factors, and statistical and econometric methods of data analysis. His recent work is highlighted by a series of studies leveraging naturalistic driving data to better understand driver behavior precipitating crash and near-crash events. Dr. Savolainen is a registered professional engineer in the state of Michigan.

CRediT statement: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing—review & editing.


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How to Cite

Gupta, N., Megat Johari, M. U., Jashami, H., & Savolainen, P. (2022). How is traffic safety affected by changes in traffic speeds following speed limit increases? An evaluation with probe vehicle data. Traffic Safety Research, 3, 000017.