Beltway Shuttle -- A Generic Automated Microbus Application

In many cases, stations on radial rail transit lines are located at intersections with circumferential freeways ("beltways" or "ring roads"). This allows passengers to be easily dropped off or picked up by automobile. It's sometimes the case that a major trip generator, such as a shopping mall, is located a few kilometers away along the freeway, but is not easily accessible from the station. If an Automated Microbus route could travel along the freeway right of way, it would extend the reach of the rail system to the mall. Because of the short trip length, the on-demand service the Microbus provides would more than compensate for its limited speed. At a 5 second minimum headway, 6 passenger vehicles would be able to carry up to about 3,000 passengers/hr per direction, assuming an average of 5 passengers/vehicle.

A short Microbus link running along a freeway could also connect two rail transit lines. This would increase the functionality of the entire rail system.

The issue is how to run a two-way Microbus route along an existing freeway right of way without incurring the expense of an elevated structure. Sometimes you can't. But in other cases, there is an answer that can be found in the different design of circumferential freeways in suburban areas, vs. freeways in the CBD.

When freeways are built in a CBD, they often have vertical retaining walls along each edge to minimize land use. In suburban areas, however, an earthen slope is commonly used instead. This slope is often wide enough to provide a two-way Microbus right of way without impinging on the freeway shoulder. There are a number of possible freeway designs, but the one considered here is a below-grade or at-grade freeway with arterial overpasses and diamond interchanges.

Route along freeway cut into slopeIf parts of the freeway happened to be at grade with no slope, the Microbus right of way could simply run beside the shoulder. This may or may not require the taking of additional land; however, a 15 ft right of way would only require 1.8 acres per two-way mile ((15 ft * 5280 ft/mile)/ 43560 sq ft/acre = 1.8, or 4.6m * 1000 m = 0.46 hectare per two-way km). If the freeway shoulder were bordered by a slope, the Microbus route could be cut from it. A 15 ft (4.6 m) right of way and a 25 degree slope would require a 7.0 ft (2.1 m) retaining wall.

Route along freeway passing under arterial In many cases, the structure needed to carry an intersecting arterial over the Microbus right of way would already exist. A widely used design for an arterial passing over a freeway has three rows of support columns, one in the center of the freeway, and one along the edge of each shoulder. This often leaves a triangular space outside the shoulder and under the arterial. This space is currently unused, but would generally be sufficient for two Microbus lanes. These would be raised above the sloping surface, not cut into it.

Microbus route along freeway passing through diamond interchangeA Microbus route along a freeway could pass over a diamond interchange on a bridge, but there's another option. If the on and off ramps of the interchange are close to the freeway for their entire length, they must rise to meet an arterial passing over the freeway. If the two-way Microbus route (shown in yellow at left) stayed just outside the ramps and maintained a constant elevation as it approached the arterial, it would fall below the level of the adjacent ramp, and go underground at some point. In many cases, it could simply pass under the arterial. There probably wouldn't be a conflict with utilities under the arterial, since the Microbus route would always be at or above the level of the nearby freeway.