Evaluation of Intersection Safety and Capacity Relevant to Signal Timing Design on Arizona's State Highway System

The Arizona Department of Transportation has determined that a comprehensive evaluation of intersection safety and capacity is necessary to address issues with signal timing design along the Arizona state highway system. Feedback has indicated that driver decisions may be affected by the length of all-red times, possibly contributing to Phoenix’s high ranking in the national statistics on red-light running fatalities. Signal timing design also is vital to intersection capacity and overall efficiency along traffic corridors. There are issues to consider with four different calculations within signal timing: 1. All-red times after protected-only left-turn phases: Longer all-red times may be seen as opportunities to run red lights, leading to crashes. Longer wait times at signals may be reducing intersection capacity, affecting safety along the traffic corridor. 2. All-red time for through movements: This phase may not be set long enough for certain intersections with relatively higher speeds, which may be contributing to safety concerns. 3. Pedestrian clearance intervals: Pedestrian crossing distance is not consistently applied from intersection to intersection. 4. Yellow change intervals: ADOT guidelines give a standard calculation that may have unintended impacts on intersection safety and capacity relevant to signal timing recommendations for other phases. A comprehensive evaluation of current signal timing design with focus on all-red time, yellow change interval and pedestrian clearance interval is needed. Research Objectives: The primary objectives of the proposed study includes evaluation of ADOT’s current signal timing design guidelines with focus on all-red phases, the yellow change interval, and the pedestrian clearance interval; determination of optimal signal timing design for the Arizona State Highway System; and a summary of findings with regards to current guidelines.