Yellow and red intervals to improve signal timing plans for left-turn movement.

The signal interval between conflicting traffic movements has long been an important operational problem, in which the human comfort and perception factors, the traffic safety, the vehicles’ driving behavior, the interpretation of the traffic laws, and the signal timing settings are all framed together. The stop/proceed dilemma for motorists approaching a signal-controlled intersection has been studied, and the appropriate method for setting the yellow change and red clearance intervals for straight movements has been developed. However, the yellow change and red clearance intervals for turning movements are yet understood in both theory and practice. In practice, a yellow change interval followed by a red clearance interval is utilized for left-turn movement in order to avoid potential conflicts with other straight or turning movements depending on the phase structure for the intersection. The lack of an appropriate method in determining the left-turn yellow change and red clearance intervals results in signal timings, which are either unsafe for left-turn vehicles or inefficient for the intersection. The primary objective of this project is to develop and test an analytical framework for setting yellow change and red clearance intervals for the left-turn movement, which integrates a comprehensive set of parameters related to safety, perception, human comfort, driver’s behavior, traffic ordinances, and intersection geometric characteristics. A systematic calibration approach is developed, which can be used directly by the field traffic engineers in selecting the appropriate values for parameters in the proposed framework. Field data are collected from a collection of intersections selected from Texas cities with diverse geographical locations, geometric characteristics, signal timing settings, drivers’ composites, and traffic conditions. The framework is calibrated using the field collected data as well as the historical accident data, which are identified from the state, cities, or counties. A portion of the field-collected data is also used to compare the calculated yellow change and red clearance intervals with the field observations. Specific procedures for the field applications of the proposed framework are specified. It is expected that the field implementation of the proposed framework will greatly improve the left-turn movement safety and enhance the general operational efficiency at intersections. (Author/publisher)