Testing alternative treatments for underused carpool lanes on narrow freeways

Abstract

Carpool lanes that are not filled to capacity can spell trouble, particularly when deployed on narrow freeways with only 3 lanes in each travel direction. Two alternative treatments for underused carpool lanes were evaluated with this in mind. Both can boost discharge flows by opening a portion of the carpool lane to all freeway traffic in the vicinity of a bottleneck. The simpler of the two alternatives opens the lane segment on a permanent basis. The other does so intermittently. Both alternatives leave the carpool lane intact on segments well upstream of an active bottleneck. Carpools are thus still enabled to bypass slower-moving queues in a freeway’s regular-use lanes. Both alternatives have previously been proposed, but were yet to be tested. The present tests entail simulations of a real 3-lane freeway. The simulation model was calibrated to roughly match unfavorable conditions presently triggered by the site’s underused carpool lane. Experiments were thereafter performed by varying travel demands parametrically. These tests unveiled why the alternatives are likely to be better options than rescinding the site’s carpool restriction and opening the full length of the lane to all traffic. Simulations also show that, unless the carpool lane is nearly filled to capacity, the alternatives perform better than does the traditional practice of running the lane through a bottleneck. The simpler of the two alternatives performed well when the carpool lane operated at less than two-thirds its capacity. The intermittent treatment was found superior when carpool demands were higher. The savings in Vehicle Hours Traveled (VHT) and People Hours Traveled (PHT) generated by the alternatives were often large. Deploying the simpler alternative under the site’s present-day conditions, for example, was found to reduce PHT and VHT by more than 1,000 (hours) each, over a 5-h period spanning the afternoon rush. These improvements did not come at great cost to carpools. To the contrary, carpools often benefited from the alternative treatments, despite having at times to traverse the site’s bottleneck sans priority. This is because by diminishing regular-lane queues, the alternatives also diminished the deleterious friction effect on carpool-lane speeds. Policy implications for carpool- and other kinds of special-use lanes are discussed.