The paper explores some of the impacts of setting aside road lanes for the exclusive use of select vehicle classes. We examine first the case of lanes that are reserved for carpools, and then extend the analysis to bus-only lanes. In doing so, the paper makes three contributions. The first is methodological: it illustrates the importance of analyzing freeway data in full spatiotemporal detail. The second is physical: data reveal that carpool lanes are not as damaging as previously reported. In fact, these lanes are found to smooth traffic in adjacent lanes so much (by diminishing disruptive vehicle interactions near bottlenecks) that even substantially underutilized carpool lanes can increase bottleneck discharge flows. The third contribution is theoretical: it uses the smoothing phenomenon to show how the judicious deployment of bus-only lanes on freeways and city streets can favorably affect not just buses, but also cars.
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