Disadvantages
Re-routings, temporary or permanent, are not usually readily available outside of "downtown" areas where the buses may be re-routed via adjacent business area streets where other trolleybus routes operate. This problem was highlighted in Vancouver in July 2008, when an explosion closed several roads in the city's downtown core. Because of the closure, trolleys were forced to detour several kilometers off their route in order to stay on the wires, leaving major portions of their routes unserved and service well off schedule. Also trolley buses have unique operating characteristics, such as trolley drivers must slow down at turns and through switches in the overhead wire system.
Some trolleybus systems have been criticised for aesthetic reasons, with city residents complaining that the jumble of overhead wires was unsightly. Intersections often have a "webbed ceiling" appearance, due to multiple crossing and converging sets of trolley wires.
Dewirements — when the trolley poles come off of the wires — sometimes occur, especially in areas subject to heavy snow. After a dewirement, trolleybuses not equipped with an auxiliary power unit (APU) are stranded without power. However, dewirements are relatively rare in modern systems with well-maintained overhead wires, hangers, fittings and "contact shoes". Trolleybuses are equipped with special insulated pole ropes which drivers use to reconnect the trolley poles with the overhead wires in case of dewirement. When approaching switches, trolleybuses usually must decelerate in order to avoid dewiring, and this deceleration can potentially add slightly to traffic congestion.
Trolleybuses cannot overtake one another in regular service unless two separate sets of wires with a switch are provided or the vehicles are equipped with off-wire capability, but the latter is an increasingly common feature of new trolleybuses.
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