Dual-mode bus

A dual-mode bus is a bus that can run independently on power from two different sources, typically electricity from overhead lines (in a similar way to trolleybuses) or batteries, alternated with conventional fossil fuel (generally diesel fuel).

In contrast to other hybrid buses, dual-mode buses can run forever exclusively on their electric power source (wires). Several of the examples listed below involve the use of dual-mode buses to travel through a tunnel on electric overhead power.

Many modern trolleybuses are equipped with auxiliary propulsion systems, either using a small diesel engine or battery power, allowing movement away from the overhead wires, called "off-wire" movement, but such vehicles are generally not considered to be dual-mode buses if their off-wire capability is very limited. Examples include the fleet of about 300 trolleybuses in San Francisco and the trolleybuses used on a 2005-opened system in Rome, Italy, which are capable of running on battery power only for short distances or short periods of time before needing recharging. The Rome vehicles are powered from overhead trolley wires over most of the 11.5-km route and only use battery power on the 500-metre section closest to the city centre.

Dual-mode vehicle

A dual-mode vehicle is a vehicle that can run on conventional road surfaces or a dedicated track known as a "guideway". Dual-mode vehicles are commonly electrically powered and run in dual-mode for power too, using batteries for short distance and low speeds, and track-fed power for longer distances and higher speeds. Dual-mode vehicles were originally studied as a way to make electric cars suitable for inter-city travel without the need for a separate engine. More recently, starting in the 1990s, a number of dual-mode mass transit systems have appeared, most notably a number of rubber tyred trams and guided buses.

The problem

The modern automobile is one of the most flexible forms of transit invented. It can operate in almost any weather, is instantly available on demand, carries hundreds to thousands of pounds of cargo and drives directly from origin to destination. The modern internal combustion engine is able to provide power with relatively good efficiency over a wide range of power demands. The fuel is simple to store and transport, is energy dense, and can re-fuel a vehicle in moments. This combination of features means that a car can be used to travel over any sort of distance, from inter-city trips at low-speed for shopping, to inter-city trips at high-speeds to visit distant relatives. No other form of transit offers the same variety of capabilities.