Metropolitan areas in the US are well served by networks of radial and circumferential high-performance limited-access highways ("expressways"). Express bus routes were in many cases routed to these facilities soon after they were opened to traffic. Many express routes, "Freeway Flyers," as they were called in Los Angeles, are designed for long-distance commuters: they often provide service in the peak only, seated passengers only, and a premium fare.
Although these services are often quite successful, they are subject to limitations. First, there is no provision made for intermediate stops between residential collection and CBD distribution. Second, peak period traffic congestion can make service slow and unpredictable.
The seminal report, Bus Use of Highways (NCHRP reports 143 and 155) by Herbert Levinson and colleagues, covers a broad range of design and operational issues with regard to taking maximum advantage of expressways to improve public transit service. The 1998 NCHRP report, HOV Systems Design Manual, updates this work. Some of the tactics used to speed expressway bus service include the following:
Bus-only or HOV lanes at toll plazas, on those urban expressways which charge tolls;
Queue by-pass lanes where ramp metering has been introduced;
Bus-only lanes on expressways;
High-occupancy vehicle (HOV) lanes, typically open to all vehicles with a minimum of either two or three occupants.
There are many different design alternatives for HOV lanes. They can be two-directional, or they can be single-direction, reversible facilities, in corridors with imbalances between directions of flow. HOV lanes can be physically separated from other lanes, or they can be separated only with paint and signage. Concurrent flow lanes can either be on the left or on the shoulder.
Reversible facilities take advantage of imbalances in directional flows by time of day. Some consist of one or two lanes constructed in the median of an expressway. Another option is to use one of the wrong direction lanes to create a contraflow lane, such as the Lincoln Tunnel XBL. Contraflow lanes typically have a physical divider, such as stantions or cones, to separate permitted users from other direction traffic. A newer technology is the use of movable concrete barriers. These barriers can be moved across the roadway by a special truck in between hours of operation. Examples of these "zipper" lanes can be found in Boston and Dallas.
Bus on expressway service is generally more suitable for express operations, that is, where a bus accumulates passengers and then enters the expressway to operate non-stop to the CBD. The lack of on-line stations makes it difficult to offer intermediate stops. However, it is possible to retrofit stations in existing expressway medians.
A high-performance bus system can include a combination of bus use of HOV lanes, uncongested expressway lanes, or busways for line-haul portions of the route. For arterial and CBD portions of a route, the techniques for improving service described below can be used. Such a system can be developed and upgraded incrementally.