Calculating Approach Capacity of Diverging Diamond Interchanges with Consideration of Internal Queue Effects

The diverging diamond interchange (DDI) is an innovative alternative to the conventional diamond interchange. Its operational efficiency has proved promising because its unique geometric design allows the operation of flexible signal phasing schemes. Of all DDI phasing schemes, two-phase operations can best highlight the merit of the DDI design by using overlaps. An internal queue is allowed with different types of two-phase operations. Therefore, conventional capacity calculation methods may not be appropriate for the DDI if the effects of the internal queue need to be considered. To fill this gap, a method was developed to calculate the approach capacity of the DDI by using an analytical model that would consider the impacts of the internal queue on the traffic progression of either arterial roads or freeway ramps. The proposed model was fully calibrated with microscopic traffic simulation in Vissim; comparison of the results indicates that the model has promising accuracy and stability. One of the proposed model’s most important functions is the calculation of lost green time of traffic movements of the DDI. A similar method documented in the Highway Capacity Manual 2010 (HCM 2010), Chapter 22, was also designed to calculate the lost green time of signalized interchanges due to the impact of an internal queue. A comparison of the two methods found that the HCM 2010 method is not appropriate for DDIs because the movements feeding into the internal space of a DDI are significantly different from the movements of a conventional signalized interchange.