Development of Road Diet Segment and Intersection Crash Modification Factors

Road diets can offer potential safety improvements for both pedestrians and vehicles. The additional space provided by reducing the number of vehicular through-lanes can be reallocated into other uses such as bicycle lanes, parking, sidewalks, transit use, turn lanes, curb extensions, parklets, or pedestrian refuge islands. Although several studies have examined the safety effectiveness of road diets, there are still a number of uncertainties. First, there is a great deal of variation in documented safety effects. Secondly, many of these past studies have only generated overall CMFs and have not created separate CMFs for segments and intersections. It is expected that intersection CMFs will be a critical need when evaluating future road diet safety effects given the complex interactions and conflicts present at these locations. This research addresses this gap in the literature by: 1) Providing separate CMFs for segments and intersections to account for the different conflicts in roadways 2) Providing CMFs that are specific by intersection types. This includes signalized and unsignalized intersections, and intersections with different leg configurations (3-leg and 4-leg).
This thesis evaluates the safety effectiveness of road diets in Virginia using the Empirical Bayes (EB) method, focusing on the common road diet conversion from a four-lane roadway to a three-lane roadway with added bike lanes. A total of 36 segment sites and 39 intersections were identified in Virginia where road diet installations were implemented between the years 2009 to 2018. The analysis showed segment crash modification factors (CMFs) of 0.62 for total crashes and 0.36 for fatal and injury (FI) crashes. Across all intersection types, the CMFs were 0.65 for total crashes and 0.54 for FI crashes. All CMFs were found to be statistically significant at a 95% confidence level. When intersections were separated into signalized and unsignalized intersections, no significant safety benefit was found for unsignalized intersections, however. Based on the results, it is concluded that road diets can potentially reduce crashes and public agencies should consider the safety benefits of road diets when justifying roadway improvements.
There are several recommendations for future research in this topic. First, it would be desirable to increase the sample size by including additional cross sections such as roadways that include medians, parking, and widening sidewalks to broaden the scope of road diet safety research. Second, because a good portion of the sites were implemented recently, it is important to collect more crash data in the after-period to better quantify road diet safety effects and the relationship with the geometric design and AADT. This need is particularly evident in the results that were obtained for unsignalized intersections. Future research should focus on assessing the safety effects of road diets at unsignalized intersections to determine if this thesis’s findings remain stable with the addition of more data and sites.