Short Right-Turn Lanes and Auxiliary Through Lanes: Intersection Capacity, Signal Timing, and Negative Lost Time

Some intersection approaches have right-turn lanes and/or auxiliary through lanes that are short in the sense that during the queuing process, entry to these lanes can be blocked by the queue in the adjacent continuous lane, limiting their capacity contribution. Conventional capacity and signal timing methods typically treat these lanes as full additional lanes, which can result in large errors in estimates of capacity and delay. This paper develops a consistent method for capacity analysis, signal timing design, and delay estimation at signalized intersections with short added right-turn and auxiliary through lanes. Short lanes are modeled as providing “bonus flow” per cycle, equal to their functional storage capacity, rather than a contribution to saturation flow rate. In analysis, this bonus flow is represented as negative lost time—a head start equivalent, allowing the proposed model to be implemented in conventional analysis with minor modifications. This study shows that with short added lanes, capacity per hour does not increase with cycle length as much as conventional methods predict, because the benefit of fewer switches per hour is offset by fewer green starts, thus less bonus flow. It is also possible for an intersection’s sum of critical lost time to be negative, in which case there is a unique cycle length that maximizes capacity—a short cycle with a red interval just long enough to replenish the short lane. Intersection-level capacity and saturation flow rate measures are proposed. Reservice is demonstrated to be a promising strategy, offering a capacity gain of 21% in one example.