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. The literature review revealed that little published information exists on the practice of setting posted speed limits lower than engineering recommended values. The survey was sent to all state transportation agencies with representation on the AASHTO Subcommittee on Traffic Engineering, which included a total of 71 representatives from 51 states or territories. A total 22 of the 28 responding agencies indicated that they engaged in the practice of setting speed limits lower than engineering recommendations. About half of these agencies had a policy or guidance document describing the practice. Overall, few agencies reported evaluating the changes to operating speed or safety resulting from setting speed limits lower than engineering recommendations. About half of the 28 responding agencies evaluated driver compliance with the lower posted speed limit and found that the compliance was generally poor. Operating speed data were collected at three sites with posted speed limits set 5 mph lower than engineering recommendations; two sites with posted speed limits set 10 mph lower than engineering recommendations; two sites with posted speed limits set 15 mph lower than engineering recommendations; one site with a posted speed limit set 25 mph lower than engineering recommendations; and, four comparison sites with posted speed limits set equal to the engineering recommended values. Data were collected from each site on three unique days: one with no speed enforcement present; one with light speed enforcement present; and, one with heavy speed enforcement present. Statistical models were developed to describe mean operating speeds, 85th percentile operating speeds and driver compliance with posted speed limits. The operating speed evaluation produced results that were consistent with other state transportation agency experiences when setting posted speed limits lower than engineering recommendations. When the posted speed limit was set only 5 mph lower than the engineering posted speed limit, drivers tended to more closely comply with the posted speed limit. Compliance tended to lessen as the difference between the engineering recommended posted speed limit and the posted speed limit increased. When the posted speed limit was set 15 to 25 mph lower than the engineering recommended speed limit, there appeared to be a low level of compliance with the posted speed limit. 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 pproximately 4 mph. Additionally, known heavy enforcement increased the odds that drivers would comply with the posted speed limit. The safety evaluation included reported crash frequency data from six sites with posted speed limits set 5 mph lower than engineering recommendations; five sites with posted speed limits set 10 mph lower than engineering recommendations; two sites with posted speed limits set 15 mph lower than engineering recommendations; and, one site with a posted speed limit set 25 mph lower than engineering recommendations. The research team used the empirical Bayes (EB) before-after approach to develop Crash Modification Factors (CMFs) to describe the expected change in crash frequency when setting posted speed limits lower than engineering recommendations. The proposed EB analysis properly accounts for statistical factors such as: regression-to-the-mean, differences in traffic volume, and crash trends (time series effects) between the periods before and after posted speed limits were set lower than engineering recommendations. While data were only available for a handful of sites that implemented this practice, the beforeafter analysis found that there is a statistically significant reduction in total and fatal + injury crashes at locations with posted speed limits set 5 mph lower than engineering recommendations. Locations with posted speed limits set 10 mph lower than engineering recommendations experienced a decrease in total crash frequency but an increase in fatal + injury crash frequency. The safety effects of setting speed limits 15 to 25 mph lower than engineering recommendations is less clear as the results were not statistically significant, likely due to the small sample of sites included in the evaluation. (Author/publisher)
Samenvatting