Speed Limits Set Lower than Engineering Recommendations

  • Published: Pennsylvania State University, 2016
  • Authors: Donnell, E. T., Gayah, V. V., Yu, Z., Li, L., DePrator, A.
  • Date Added: 05 May 2017
  • Last Update: 05 May 2017
  • Format: pdf


The purpose of this project is to provide the Montana Department of Transportation (MDT) with objective information concerning the operational and safety impacts of setting posted speed limits lower than engineering recommended values. This practice has been used on Montana roadways for a variety of reasons, but the safety and operational impacts are largely unknown. The project involved four unique components: a comprehensive literature review, a survey of other state transportation agencies, collection of speed and safety data from a variety of Montana roadways, and an analysis of these data.


An Empirical Bayes observational before-after study.

Key Findings:

  • There is a statistically significant reduction in total and fatal and injury crashes at sites with engineering speed limits set 5 mph lower than engineering recommendations.

  • At locations with posted speed limits set 10 mph lower than engineering recommendations, there was a decrease in total crash frequency, but an increase in fatal and injury crash frequency.

  • The safety effects of setting posted speed limits 15 or 25 mph lower than engineering recommendations is less clear, because the results were not statistically significant, due to the small sample size of sites available for inclusion in the analysis.

  • When the posted speed limit was set only 5 mph lower than the engineering posted speed limit, drivers tend to more closely comply with the posted speed limit.

  • Compliance tends to lessen as the difference between the engineering recommended posted speed limit and the posted speed limit increases.

  • The practice of light enforcement, which was defined as highway patrol vehicles making frequent passes through locations with posted speed limits set lower than engineering recommendations, appeared to have only a nominal effect on vehicle operating speeds.

  • Known heavy enforcement, defined as a stationary highway patrol vehicle present within the speed zone, reduced mean and 85th-percentile vehicle operating speeds by approximately 4 mph.


Before and after studies; Compliance; Crash injuries; Crash rates; Fatalities; Operating speed; Speed limits; Traffic law enforcement; Traffic safety.