Saturation headways and lost times at signalized intersections

Roadway intersections are often the focal points of conflicts and congestion in the overall roadway network. At the same time, the operation of intersections is often an indicator of the efficiency of an urban street system. Capacity, particularly at signalized intersections, is a key indicator of intersection operation, and has been the subject of much research in recent years. The capacity at a signalized intersection is a function of saturation headway and lost time. Therefore, it is most important to determine saturation headway and lost time values in conducting capacity analyses at signalized intersections. Saturation headway and lost time are also required to set signal timings. Saturation headway and lost time can be directly measured from field data, but there are no established rules among traffic engineers on how to measure them. The engineers establish their own rules relying on observational data, then use this data to estimate saturation headway and lost time.; This study provides the mathematical methods for directly measuring saturation headway and lost time values to achieve reliable results. In this study, a mathematical model, which represented well the queue discharge headways, was selected and tested using field data, then verified as a good model by providing reasonable saturation headways and lost times.; A total of ten level approach lanes at eight signalized intersections were selected for this study and only straight through passenger cars were analyzed to test the selected model. The tests as conducted verified that the model was acceptable, and the model generated good saturation headway and lost time values for typical signalized intersection approaches. The average saturation headway was 1.91 seconds, which is about 0.1 seconds less than the currently used default value (2.0 seconds) under ideal conditions. The average starting and stopping lost times were 2.57 and 2.56 seconds, respectively, and the average total lost time was 5.13 seconds, which is greater than the currently accepted values (3-5 seconds).; This method is simple, reasonable, and estimates good saturation headway and lost time values. It forces traffic engineers to obtain more reliable input data which are used when using computer models to perform capacity analyses at signalized intersections.