Reginald Souleyrettereg@iastate.edu email >
Associate Director for Transportation Planning and Information Systems
About the research
Crashes at signalized intersections account for approximately 20% of all crashes both nationally and within the state of Minnesota. Past research suggests that the use of all-red clearance interval at signalized intersections may reduce intersection crashes, particularly those related to signal violations. Other research suggests that this reduction may only be temporary. This research evaluated the safety effect of all-red clearance intervals at low speed urban 4 way intersections in the City of Minneapolis. The study included a review of literature and assessment of Midwestern state and local practice related to the use of all-red phasing. A cross-section analysis using four years of data was presented, which does not substantiate any safety benefit of all-red phasing at study area intersections. Several regression models (generalized linear mixed models with Poisson error distribution and log link function and linear mixed models with transformed data) were also presented. The models also point to no safety benefit. A before and after analysis using 11 years of data was conducted to evaluate both short and long term effects. While results indicated short-term reductions in crash rates (approximately one year after the implementation), long-term reductions were not observed.