Effects of HOV lanes on freeway bottlenecks

High occupancy vehicle (HOV) lanes are restricted-use freeway lanes reserved for vehicles with more than a predetermined number of occupants. This paper examines the physics of HOV lanes placed on median lanes, with open access everywhere. HOV lanes can affect the capacity of freeway bottlenecks through both an under-utilization effect and a disruption effect. An under-utilized HOV lane passing through a bottleneck obviously discharges less flow than possible. But lane changes in and out of the HOV lane can also disrupt the flow on the adjacent general purpose (GP) lanes, and reduce their discharge rate as well. Bottleneck capacity reductions arising from the combination of both effects are undesirable because they increase vehicle-hours of travel. This paper shows with systematic simulations that the disruption effect is not noticeable at isolated bottlenecks. If anything, HOV lanes seem to have a smoothing effect that increases the GP discharge flows at these locations. Reductions in GP-capacity were found only in highly idealized situations without bottlenecks. Thus, it appears that the total discharge rate of bottlenecks with and without HOV lanes can be conservatively analyzed assuming that the capacity of the GP lanes is not affected by the HOV lane. Using this assumption, the paper then shows how to estimate total bottleneck capacity, how to diagnose problems with existing HOV lanes, and how to deploy new ones without creating new bottlenecks or changing the total flow through existing ones. This milestone is sometimes sufficient to guarantee no change in the total vehicle-hours of travel and, hence, a reduction in people-hours of delay. The paper also examines a dynamic operating strategy for HOV lanes that increases a bottleneck's total discharge rate by exploiting the smoothing effect. This suggests that dynamic strategies can be used to reduce not just people-hours of delay but also vehicle-hours and their externalities.