Evaluation of Posted Speed Limits Reductions on Urban Roads with a High Percentage of Cyclists

Abstract

This paper presents a before and after analysis of the impact of posted speed limit (PSL) changes on passenger car (FHWA class 2 vehicle) speeds in Portland, Oregon. The study focuses on urban roads, comparing sites that underwent a PSL 5-mph reduction (treatment sites) and sites where the PSL did not change (control sites). Sites with a high percentage of and priority for cyclists (neighborhood greenways) and sites with a more standard traffic composition were compared. Differences in speed characteristics such as mean and 85th percentile speeds, the speed variance, and the proportion of vehicles exceeding a speed threshold (relative to the PSL) were evaluated on aggregate and individual scales. A series of statistical hypothesis tests were employed to assess changes in the speed characteristics among individual data set pairs. The results suggest distinct differences between the treatment and control groups and neighborhood greenway and non-greenway sites. Although there is a high degree of variability, the treatment group experienced more decreases in the speed characteristics, and by a greater amount than the control group, on average. Within the treatment group, sites with a priority for cyclists were even more likely to experience a larger reduction in operating speeds. These results could be interpreted as link-level data providing indirect yet supporting evidence for the safety in numbers hypothesis and changes in motorists’ behavior on neighborhood greenways.