Impact attenuator

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. They may also be designed to redirect the vehicle away from the hazard or away from roadway machinery and workers. Impact attenuators are usually placed in front of fixed structures near highways, such as gore points, crash barrier introductions, or overpass supports. Temporary versions may be used for road construction projects.

Truck-mounted versions, similar in some ways to railcar buffers, can be deployed on vehicles that are prone to being struck from behind, such as snow plows and road construction or maintenance vehicles. Work zone regulations often specify a minimum buffer distance between the attenuator truck and the work area, and a minimum mass for the truck, to minimize the chances that the truck will be pushed forward by a crash into the workers or machinery. This is especially important in mobile work zones where the truck's parking brake may not be engaged. The truck may also be in movement (albeit slower than the vehicles that could crash into it).

Impact attenuators can be categorized by the method used to dissipate kinetic energy:

In the United States, impact attenuators are tested and classified according to AASHTO Manual for Assessing Safety Hardware (MASH), which superseded the Federal Highway Administration NCHRP Report 350. Classification is based on the maximum speed of a vehicle during a collision for which the attenuator is designed.

Gating impact attenuators allow vehicles impacting from the side to pass through them (akin to a gate).

Gating attenuators are more economical, but require a greater clear space surrounding them to be effective; without enough space, errant cars may pass through into another hazard, such as into lanes of opposing traffic.

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