Utilization of Double Left-Turn Lanes with Downstream Lane Reductions

The capacity of left-turn movements at signalized intersections is sometimes increased by using two exclusive double left-turn lanes at the intersection followed by a reduction to a single lane after the turn is made. A typical application is the use of double left-turn lanes onto a freeway entrance ramp, where the right lane is dropped in advance of the merge area. The effectiveness of the “added” outside left-turn lane and downstream auxiliary lane depends on the amount of traffic using that lane. While equal distribution of traffic between a continuous and an auxiliary through lane would result in the greatest total capacity of this lane pair, traffic and geometric factors are usually such that this does not occur. The 1994 Highway Capacity Manual does not address intersection configurations with downstream lane reductions. This paper describes the development of a mathematical model, based on field data, which does predict auxiliary lane use for this intersection configuration.