Taxonomy and description

Guided bus systems provide a lower cost alternative to light rail while having the advantages of dedicated rights of way. While totally separate rights of way can be provided, as in Adelaide, most current proposals envisage providing guideways solely where buses need to bypass congestion, as in Leeds. This can be achieved with minimal space requirements; the guideway need only be 3m wide, and is only needed in the direction in which congestion is experienced. Specially equipped buses can then operate normally on the rest of their routes, hence providing much more extensive suburban coverage than light rail (Read, Allport and Buchanan, 1990). They have a number of objectives, including to increase the reliability, speed and accessibility of bus services, to reduce road congestion through the diversion of car journeys to bus and to generate increased revenue through increased bus-use.

Terminology
Guided bus systems involve taking the steering of the bus off the bus and away from the bus driver for all or, more usually, part of the route. In doing so, they eliminate the need to allow for any lateral movement of the bus within a lane of traffic. A bus is generally approximately 2.5m wide, but a bus lane is usually 3.75m or even 4m wide to allow for this lateral movement. A guided bus system, therefore, provides opportunities to implement dedicated busways where road space is in short supply and, hence, where conventional bus lanes could be impractical. Depending upon the guidance technology used, systems may also provide opportunities to improve physical access to the bus by minimising the vertical and horizontal gaps between the bus stop and the bus. Some systems also provide for physical segregation from other traffic, meaning that it is impossible for other vehicles to block the guideway. They also provide for considerable flexibility in operations, in that a suitably adapted bus can travel on a guideway where this is available but can also travel on any other part of the road network as required.

Types of Guided Bus System

Whilst kerb guidance is the most commonly used system, there are a range of systems available or in development:

Kerb guided Systems - to date, this is the most commonly used guidance technology. Specially equipped buses, with small 'guidewheels' positioned in front of the main front wheels, are guided along a track formed out of two vertical upstands (or kerbs), separated by the width of a bus axle (approximately 2.5m). On entering the track (or guideway), the guidewheels connect with the kerbs and guide the bus.


Automatic Electronic Guidance Systems - specially adapted buses may be guided electronically via underground cables which transmit signals to the vehicle (as with the electronically guided Mercedes service vehicles operating in the Channel Tunnel). However, a recent investigation of this form of guidance system raised a number of safety concerns (Bain, 2002);

Optical guidance - this is a more recent development in guidance technology and involves dashboard- mounted cameras, a video- monitoring system and a road- marking recognition system. Field trials of such a system took place in 2001 under the CIVIS project and there are plans to implement a test scheme in Las Vegas.

Other guided bus systems include the Translohr vehicle, which uses two front rollers running on a central guideway to steer the vehicle, and Bombardier's low floor guided light tram, for which automatic vehicle guidance is effected via small metal wheels which run in a central groove in the pavement. However, in the two instances where guided light tram has been implemented - in Caen and Nancy - safety concerns regarding the transfer from automatic guidance to manual steering have resulted in the systems being closed pending a technical enquiry (Bain, 2002).

Technology
There are two aspects to the technology of kerb guided bus: the guideway itself and the on-bus equipment.

The guideway - Typically, the guideway has two tracks along which the bus' wheels run. These tracks are typically made from reinforced concrete as the wear resulting from the wheel path being exactly the same each and every time would erode other surfaces, eg tarmac, much more quickly. At the outer edge of each of the two tracks is a vertical kerb. The two kerbs provide the guidance to the bus and are set 2.6m apart from one another. A drainage channel of approximately 1.2m runs between the two tracks.

The on-bus equipment - The only equipment required to modify a standard bus, with standard steering, for operations along the guideway are the guidewheels. The guidewheels use 180mm solid rubber tyres and are positioned in front of the main front wheels of the bus. They are installed via a J beam which is bolted to the back of the front wheel assembly and protrude 5cm out from the edge of the bus on each side.