EVALUATION OF HIGH-SPEED ISOLATED SIGNALIZED INTERSECTIONS IN CALIFORNIA. FINAL REPORT

High-speed isolated signalized intersections (HSISI's) are generally encountered after long uninterrupted and uncongested flow conditions, thereby catching some motorists by surprise. For this reason alone, approaches to HSISI's need to have safe and effective warning, control, and intersection geometric treatments. This research identified characteristics at California HSISI's that relate to accident rates. A data base consisting of approximately 100 intersection and approach variables was established and used to investigate advance warning, signal timing and phasing, channelization, signal equipment configurations, shoulder widths and types, median widths and types, and approach speeds. Forty HSISI's out of the approximate 100 statewide were chosen for analysis. Twenty were selected from the highest accident-rate group and twenty from the lowest. Statistical analysis identified relationships between approach variables and approach accident rates. The primary variables found to be significantly correlated to low accident rates on approaches to HSISI's were: presence of an advance warning sign with a flashing beacon, presence of a separate left-turn phase, presence of a raised median, and wide paved shoulders. Eight approaches to HSISI's were identified for the possibility of implementing suggested improvements and for use in before and after studies.