Impact of speed limit changes on urban streets

Principal Investigator(s):

Gary Davis, Professor, Civil, Environmental and Geo-Engineering


  • John Hourdos, Former Research Associate Professor, Civil, Environmental and Geo-Engineering

Project summary:

In 2019 the Minnesota Legislature amended that state's statutes to allow cities to set speed limits on city-owned streets. This project used this change in speed-limit law as an opportunity to study the effect of speed-limit changes on vehicle speeds. In February 2021, researchers surveyed 33 cities within the Twin Cities metro area and identified the city of St. Louis Park as planning to implement a city-wide change in speed limits, with a default speed limit of 20 mph but with selected roads being signed for limits ranging from 25 mph to 35 mph. The city was chosen for a before-and-after study of how changes in speed limits affect vehicle speeds. Speed data was collected using road tube traffic recorders in the summer of 2021 (two to four months before the speed limit change) and in the summer of 2022 (six to eight months after the change). There was considerable variability regarding what was seen at individual locations, with before/after differences in mean speed ranging from a decrease of 7 mph to an increase of 2.4 mph. On average, mean speeds were slightly lower (1-2 mph) in the after period, both on streets where the speed limit was lowered and on streets where the limit was unchanged. This pattern, modest reductions in mean speeds following a reduction in speed limit, with possible spillover, was consistent with what has been seen in other cities in North America and Great Britain.

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