The signalised intersection methodology presented in the 1985 Highway Capacity Manual (HCM) introduced a new delay model, which naturally invites comparison with the delay models contained in existing traffic signal timing design and analysis techniques. This paper compares the HCM delay computations with those performed by the Signal Operations Analysis Package (SOAP) and by TRANSYT-7F Release 5. The paper focuses on the effect of the degree of saturation, the peak-hour factor, and the period length on delay computations and on the treatment of left turns opposed by oncoming traffic. All of the models agreed closely at volume levels below the saturation point. When conditions became oversaturated, the models diverged; however, they could be made to agree by the proper choice of parameters. The computed saturation flow rates for left turns opposed by oncoming traffic also agreed closely. However, the treatment of protected plus permitted left turns produced substantial differences. It was concluded that neither SOAP nor the HCM treats this case adequately. Therefore, an alternative model based on a deterministic queuing process was proposed and evaluated.
Abstract