EVALUATION OF SEATTLE FLASHING YELLOW PROTECTED/PERMISSIVE LEFT-TURN SIGNAL DISPLAY

The trade-off between safety and efficiency of left-turn movements at signalized intersections remains an on-going issue for signal practitioners. In particular, the selection of the protected/permissive left-turn operation represents the epitome of this trade-off. The protected/permissive strategy is the preferred treatment for left-turn operation requiring protection for reasons other than accident reduction. Upon determination that this strategy is appropriate to the field conditions, the standard Manual of Uniform Traffic Control Devices (MUTCD) four or five section signal display is applied in accordance with agency policy. This display is normally augmented by verbal signing to clarify the left-turning driver's obligation during the circular green indication. One alternative or novel approach to the display and signing has been applied by the City of Seattle. This alternative in device application is the use of a four section display (fiber optic arrow), with the third section (from top) being a circular flashing yellow. The operational sequence is standard except for the utilization of the flashing yellow in place of the circular green during the permissive interval. The permissive period terminates with a standard yellow change interval whereby the flashing yellow section goes dark and the second section goes from dark to steady yellow. A standard red clearance interval follows. The flashing yellow application did not require revision to the existing traffic code. The effectiveness of the flashing yellow signal display in reducing accidents shall be contrasted with the standard MUTCD applications as deployed in the surrounding jurisdictions.