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 on declines, and help prevent the vehicle from running away by accelerating down the decline.They are not usually capable of bringing vehicles to a standstill, as their effectiveness diminishes as vehicle speed lowers.hey are usually used as an additional assistance to slow vehicles, with the final braking done by a conventional friction braking system. As the friction brake will be used less, particularly at higher speeds, their service life is increased.