Abstract

Most special-use freeway lanes in the US, whether reserved for carpools, toll-paying commuters or both, are physically separated from the adjacent regular-use lanes by some form of barrier. Vehicle movements in and out of a special lane of this type are permitted only at select access points along the route. The barrier at each select point might open for a distance of 400 m or so. Limiting access in this way is said to reduce the “turbulence†that might otherwise occur were the special lane not to have a buffer, such that vehicles could instead enter or exit that lane anywhere along its length. Yet, real freeway traffic studied in spatiotemporal fashion shows that access points are prone to become bottlenecks. The problem occurs when traffic in the regular lanes becomes dense, as commonly happens during a rush. Drivers then seek refuge in the special lane in greater numbers. Since the vehicular maneuvers through the access point are focused within a limited physical space, they can become disruptive and further degrade traffic. Degradation can occur both in the special lane and in the adjacent regular ones. The damage can be worse than what occurs when barrier are not used to limit special-lane ingress and egress. Policy implications are discussed.

Abstract

High Occupancy Vehicle (HOV) lanes and ramp metering have been implemented at several places in the country to improve traffic conditions on freeways. This report presents a comprehensive study of the state of the art and state of the practice of these facilities in the U.S. and their effectiveness in improving traffic conditions on the freeway. The various types of HOV facilities have been discussed along with other system components such as park-and-ride facilities, ridership programs, enforcement areas and policies, ingress/egress facilities. The cost of several HOV facilities in the country has been presented and a set of guidelines have been prepared that may be followed for the implementation of a HOV Lane Facility. A comprehensive review of the various ramp metering facilities in the country has been presented. The components of the ramp metering system, the physical infrastructure and the various classifications of the ramp metering algorithms have been discussed. The results of a feasibility study for the implementation of these facilities on I-70 in Indianapolis, between I-65 and I-465 have also been presented.