Optimal Discharge Speed and Queue Discharge Headway at Signalized Intersections

In academia research regarding the intersection saturation headway, there are various and sometimes seemingly conflicting findings in terms of how the discharge headway or saturation flow rate changes as green time elapses. Some research found the discharge headways remained constant; some found the discharge headways had a compression trend. Others found the discharge headways got longer after a certain point of time as the discharge continued. This paper introduces an optimal discharge speed concept to explain the different trends found in saturation headway research. For a queue of traffic to discharge from an intersection, if the discharge speed quickly reached the maximum discharge speed which was at or below the optimal discharge speed, the discharge headways would remain constant; if the discharge speeds continued to increase but never exceeded the optimal speed in the whole discharge process, the headway would show a compression trend; if the discharge speeds at a certain point exceeded the optimal discharge speed, the headway would show an elongation trend. The queue discharge characteristics at one signalized intersection located in Toledo, Ohio were studied. The discharge headways of through lane traffic demonstrated an elongation trend. The optimal discharge speed that was associated with the lowest average headway was 27 mph.