Dual Influences on Vehicle Speeds in Special-Use Lanes and Policy Implications

Slow speeds in a special-use lane, such as a carpool (HOV) or bus lane, can be due to both high demand for that lane and slow speeds in the adjacent regular-use lane. These dual influences are confirmed from months of data collected from all freeway carpool facilities in the San Francisco Bay Area. Both influences hold for other types of special-use lanes, including bus lanes. New US regulation stipulating that most classes of low-emitting vehicles, or LEVs, be banned from slow-moving carpool lanes. While LEVs invariably constitute only about 1 percent of the freeway traffic demand in the San Francisco Bay Area, forcing some or all of these vehicles to regular-use lanes can significantly add to regular-lane congestion, and that this, in turn, can also be damaging to vehicles that continue to use the carpool lanes. Counterproductive outcomes of this kind are predicted first by applying kinematic wave analysis to a real Bay Area freeway. The site stands to suffer less from the regulation than will others in the region but the site’s people-hours and vehicle-hours traveled during the rush are predicted to each increase by more than 10 percent and that carpool-lane traffic will share in the damages. Real data from the site support these predictions. Further parametric analysis of a hypothetical, but more generic freeway system indicates that these kinds of negative outcomes will be widespread. Constructive ways to amend the new regulation are discussed, as are promising strategies to increase the vehicle speeds in carpool lanes by improving the travel conditions in regular lanes.

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