Impact attenuator. A Truck-Mounted Attenuator (TMA) in Auckland. An impact attenuator, also known as a Truck Mounted Attenuator (TMA), crash cushion, crash attenuator, or cowboy cushion, is a device intended to reduce the damage to structures, vehicles, and motorists resulting from a motor vehicle collision. Impact attenuators are designed to absorb the colliding vehicle's kinetic energy.