Through numerous field observations at 5 signalized intersections in the center of Shanghai City, this paper studied the reliability of signalized intersection capacity. The volatility of the intersections' headway was described firstly. And then, 1500 signal circles data of the saturation headway of left-turn lane at 5 intersections were collected by camera, and some descriptive statistics parameters of the saturation headway were investigated. Since the multiplicative inverse of saturation headway is the saturation flow rate, so the saturation flow rate data can be easily got. We found that the capacity of signalized intersection capacity yielded to normal distribution and it could be explained by a famous low of the probability theory. Then an analogy was made between the reliability of structural capacity and signalized intersection capacity, the reliability function of signalized intersection capacity was defined. Thus, the capacity and its reliability can be understood from a viewpoint of probability. The research develops detailed models to describe the signalized intersection capacity and proposed a better understanding.
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