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Their development dating to the early 1900s, the trolleybus seemed to be a natural compromise between the electric streetcar and the petroleum-powered bus. Trolleybus systems could avoid obstacles in the street which a streetcar could not, and did not require as much capital investment as a streetcar line.
Some cities, led by the Brooklyn-Manhattan Transit Corporation (BMT—New York), subscribed to the all-four concept of using buses, trolleybuses, trams (in U.S. called streetcars, trolleys or light rail) and rapid transit subway and/or elevated lines ( metros) as appropriate for routes ranging from lightly-used to heaviest trunk line. Buses and trolleybuses in particular were seen as entry systems that could later be upgraded to rail as appropriate. Although the Brooklyn system under the BMT only built one trolleybus line, other cities, notably San Francisco, California and Philadelphia, Pennsylvania, built larger systems and still maintain "all-four." If you include cable cars as another mode, San Francisco could be called "all-five," as the cable cars do provide a general transportation function in addition to being a tourist attraction.
Trolleybuses are particularly important in hilly cities, as the electric power is more effective than diesel in climbing steep hills and have better roadway adhesion than streetcars. This is especially important in San Francisco and Seattle.
Like other electric vehicleAn electric vehicle is a vehicle that is propelled by electric motors. This is the case for metros and usually trams, and for some trains (i. for some locomotives and often for multiple units), and for electric trolleybuses. The term is used in particulars, trolleybuses are often seen as more environmentally friendly than hydrocarbonIn chemistry, a hydrocarbon is an organic compound consisting only of carbon and hydrogen. They all consist of carbon backbone and atoms of hydrogen attached to that backbone, also aliphatic hydrocarbons. For example, methane ( swamp gas) is a hydrocarbon based vehicles such as buses, but the power is not "free", and instead has to be produced at centralised power plants. On the other hand, centrally-produced power has the advantage of being more efficient, not bound to a specific fuel source, and more amenable to pollution-control as a single-source supply than individual vehicles, each with its own power generation.
One advantage that is rarely used in other vehicles is that they can generate electric power from kinetic power whilst braking, a process known as regenerative brakingRegenerative braking is any technology that allows a vehicle to recapture and store part of the kinetic energy that would ordinarily be lost when braking. A simpler technology that can only convert the energy to heat but which uses similar principles is k, or going downhill.
Another area where trolleybuses enjoy a special niche is locations where hydropowerHydropower (or waterpower harnesses the energy of moving or falling water. This is usually in the form of hydroelectricity from a dam, but it can be used directly as a mechanical force. The term refers to a number of systems in which flowing water drives is abundant and cheap. Examples of this are the extensive trolleybus systems in Vancouver, British Columbia, Canada and Seattle, Washington, USA. The last city doubly benefits, due to the steep road grades near the Downtown waterfront as well as on Queen Anne, First, and Capitol Hills.