Speed Limit Change (55 mph to 60 mph) Safety Evaluation

Status:  Complete
Report Date:  07/21/2020

Summary:

The objective of this study was to evaluate the safety impacts of increasing the speed limit from 55 mph to 60 mph on two-lane, two-way state highway road segments in Minnesota. An empirical Bayes (EB) before-after analysis was used to estimate Crash Modification factors (CMFs) for both segments and intersections. The segment analysis showed a 7 percent increase in total crashes that was statistically significant, alongside insignificant increases/decreases in injury, run off road, and head on crashes. The range of most of the segment CMFs hovered close to 1 (essentially meaning that the crashes either remained unchanged or the change was minor). The intersection analysis was split into two groups (all traffic control types and thru stop control only). The aggregate CMFs for all intersections within these two groups show that most of the CMFs hovered close to 1. Analysis was also performed on four subgroups (3- and 4-leg, lighting/no lighting) within the two main intersection groups. Disaggregating the intersections into further groups led to smaller sample sizes that led to higher standard errors showing a widespread range of CMFs around 1 for the individual crash types and severities. The aggregate analysis conducted using all the segment and intersection data showed very minor increase/decrease in the total and injury crashes. This aggregate result along with before and after operating speed data from another Minnesota Department of Transportation (2019) study showing that the 85th percentile operating speed remained the same and that the mean operating speeds increased by 1 mph following the speed limit increase can lead to a conclusion that the speed limit increase from 55 mph to 60 mph had a very minor to no effect on total and injury crashes or operating speeds.

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