A Reliability-Based Framework to Assess the Impacts of Increasing Freeways’ Posted Speed Limits

Posted speed limits have a critical impact on highway safety and mobility. Increasing the posted speed limit may increase the risk of collisions and reduce the overall safety level. This study utilized a reliability-based framework to assess the safety impacts of increasing the speed limits on highways. Four highway segments were considered, including tangents, horizontal curves, and crest and sag vertical curves. For each segment, the suitable modes of noncompliance were evaluated at four speed limit scenarios (100, 110, 120, and 130 km/h) to quantify the increased risk associated with the speed limit increase. The results indicate serious negative implications of the speed limit increase, as the risk ratio corresponding to a 10 km/h increase in speed limit averaged between 1.09 and 1.75 on tangents with varying traffic volume conditions. On horizontal curves, the risk ratios of skidding and inadequate sight distance were around 1.43 and 1.73, respectively. The same 10 km/h increase in speed limits resulted in an average risk ratio of 1.74 and 1.49 for vertical crest and vertical sag curves, respectively. The framework was applied to assess a proposed increase in speed limits on a group of freeways in Ontario, Canada, from 100 km/h to 120 km/h. The results showcase the significant added risks using actual freeway parameters and characteristics collected from the field, despite the conservative road design. Furthermore, mobility assessment by means of microsimulation indicated little to no benefits of the speed limit increase in the studied section. The presented framework advocates a proactive safety approach for practitioners to evaluate speed limit changes before implementation. Additionally, the reported findings shed light on the safety risks of raising the freeway speed limits and refute the claims of enhanced mobility that are often used to promote such changes.