Smoothing Effect on Freeway Bottlenecks: Experimental Verification and Theoretical Implications

Real data show that reserving a lane for carpools on congested freeways induces a smoothing effect that is characterized by significantly higher bottleneck discharge flows (capacities) in adjacent lanes. The effect arises because disruptive vehicle lane changing diminishes in the presence of a carpool lane. The effect is reproducible across days and freeway sites: it was observed, without exception, in all cases tested; and queueing analysis shows that the effect greatly reduces the times spent traveling on a freeway. The effect is so significant, in fact, that even a severely underused carpool lane can sometimes increase a freeway bottleneck’s total discharge flow. It follows that strategies designed to induce smoothing by other means also hold promise for managing congestion, both for freeways that have carpool lanes and those that do not; and possible strategies of this kind are discussed.