The potential for variable speed control to improve safety on urban freeways

Variable speed control systems enable transportation managers to dynamically change the posted speed limit in response to prevailing traffic and/or weather conditions. Variable speed control systems have been implemented in a number of jurisdictions throughout the world. Many of these systems have been deployed to address specific safety issues, such as steep grades or frequent occurrences of adverse weather such as fog, high winds, or blowing snow. A smaller number of systems have been deployed as a more general traffic management tool and even for these systems, there is currently limited documentation describing the quantitative safety and operational impacts. Furthermore, the impacts that are reported are primarily from systems in Europe, and may not be directly transferable to North America. This paper describes a methodology developed to evaluate the safety impacts of variable speed control systems and presents the results obtained using this methodology to evaluate a candidate variable speed control system for an urban freeway in Toronto, Canada. The evaluation was conducted using a microscopic simulation model combined with a categorical crash potential model for estimating safety impacts. The keys findings from the study show that the candidate variable speed control has the potential to provide relatively large safety benefits (i.e. up to a 40% reduction in potential for crashes). Furthermore, unlike speed management techniques that rely on in-vehicle devices, variable speed control systems can be implemented with existing technologies and with the existing vehicle fleet. The study also found, however, that the safety impacts vary depending on traffic conditions; the variable speed control system logic, and the parameters within the system algorithms. Furthermore, the results are based on several important assumptions about drivers’ reaction to variable speed limits. In particular, the analysis assumed a high degree of compliance – a level likely to be achieved only through the use of automated speed enforcement. The degree to which this and other assumptions are valid, and the impact that violation of these assumptions may have on the level of safety improvements, requires additional investigation.