Right-turn-on-red volume estimation and incremental capacity models for shared lanes at signalized intersections

The Highway Capacity Manual (HCM) is one of the most widely used transportation documents in the world. The signalized intersections methodology, one of the most frequently used portions of the manual, is used to estimate capacity and average control delay for individual lane groups and for intersections as a whole. The method does not estimate control delay for vehicles permitted to turn right on red, nor does it include these vehicles in the computation of capacity. It is recognized that vehicles making right turns on red (RTORs) increase intersection capacity. Thus, capacity, delay, and level of service may not be predicted accurately when RTORs occur. For the specific case in which a shared through and right-turn lane exists and RTORs are permitted, models were developed to estimate the number of RTORs that can be expected and the additional capacity that is realized. The models are deterministic and are consistent with the HCM methodology, are easily understood by analysts, produce reasonable results, and account for the probabilistic nature of RTORs occurring from shared lane approaches. The capacity model also demonstrates that, where RTORs are permitted and do occur, the HCM method results in an underestimation of capacity and an overestimation of delay for approaches containing shared lanes.