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A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems, usually on heavy vehicles. Retarders serve to slow vehicles, or maintain a steady speed while traveling down a hill, and help prevent the vehicle from unintentional or uncontrolled acceleration when travelling on a road surface with an uneven grade. They are not usually capable of bringing vehicles to a standstill, as their effectiveness diminishes as a vehicle's speed lowers. Instead, they are typically used as an additional aid to slow vehicles, with the final braking done by a conventional friction braking system. An additional benefit retarders are capable of providing is an increase in the service life of the friction brake, as it is subsequently used less frequently, particularly at higher speeds. Additionally, air actuated brakes serve a dual role in conserving air pressure.
Friction-based braking systems are susceptible to brake fade when used extensively for continuous periods, which can be dangerous if braking performance drops below what is required to stop the vehicle: for instance, if a truck or bus is descending a long decline, and would otherwise require something such as a runaway truck ramp to stop safely. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based.
Retarders are not restricted to road motor vehicles, but may also be used in railway systems. The British prototype Advanced Passenger Train (APT) used hydraulic retarders to allow the high-speed train to stop in the same distance as standard lower speed trains, as a pure friction-based system was not viable.