EVALUATION OF HIGH-SPEED ISOLATED SIGNALIZED INTERSECTIONS IN CALIFORNIA (WITH DISCUSSION AND CLOSURE)

High-speed isolated signalized intersections (HSISIs) are generally encountered after long uninterrupted and uncongested flow conditions, therefore catching some motorists by surprise. For this reason alone, approaches to HSISIs need to have safe and effective warning, control, and intersection geometric treatments. Characteristics at California HSISIs that relate to accident rates are identified. A new safety indicator was introduced: intersection approach accident rate. Variables investigated included advance warning signs with and without flashing beacons, signal timing and phasing, channelization, signal equipment configurations, shoulder widths and types, median widths and types, and approach speeds. Forty HSISIs out of the approximately 100 statewide were chosen for the analysis. Twenty were selected from the highest accident group and 20, from the lowest accident group. 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 HSISIs were the presence of separate left-turn phase, a raised median, wide paved shoulders, and an advance warning sign with a flashing beacon. A demonstration project and interim procedures for California are encouraged.