X and Y in Traffic Signal Design

Aerial photograph of road


The paper proposes various improvements to the traffic signal design method with particular reference to the method used in Australia. The intersection degree of saturation, X is recommended for use as a simple measure of operating conditions instead of the intersection flow ratio, Y. The use of Y may be misleading especially in the case of alternative analysis and when one or more signal phases have green times which do not satisfy pedestrian or vehicle minimum green time requirements. A simple formula for calculating the intersection X value without the need to calculate phase green times is given. It is proposed that an acceptable maximum degree of saturation Xm is used instead of an upper limit on the intersection Y value and it is recommended that X_m = 0.90 is used for general signal design purposes. The paper also describes methods for (a) calculating a minimum cycle time which yields an acceptable maximum degree of saturation, (b) calculating spare intersection capacity based on the use of Xm and a maximum acceptable cycle time and (c) signal design calculations when a minimum green time constraint is not satisfied. An alternative method for calculating signal settings is also described.


AKÇELIK, R. (1978). X and Y in Traffic Signal Design. proceedings of the Ninth Australian Research Board Conference, Brisbane, August 21-25, 1978. pp 45-60.

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