An impact attenuator, also known as a 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. Operation Impact attenuators are designed to absorb the colliding vehicle's kinetic energy to bring it to a stop safely. If no impact attenuator is present, a vehicle which strikes a rigid roadside object will suddenly stop. A person inside will promptly collide with the interior of the vehicle, and that person's internal organs will collide with their chest wall, causing severe internal injuries and possibly death. By safely dissipating the vehicle's kinetic energy, impact attenuators help prevent such injuries. Impact attenuators can be categorized by the method used to dissipate kinetic energy: Momentum transfer. Many early models used successive rows of water-filled barrels or modules. Momentum is transferred to the water, reducing the speed of the impacting vehicle. Material deformation. Many newer attenuators use crushable materials (like various kinds of foam) that create a crumple zone, absorbing energy. Others flatten a corrugated steel guard rail section, or split a steel box beam. Friction. Some attenuators work by forcing a steel cable or strap through an angled slot or tube, converting kinetic energy into heat. Types Gating 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. Water-filled attenuators Water-filled attenuators consist of containers filled with water to absorb impact energy. They are typically not anchored to the ground, and therefore benefit from easy deployment and relocation using barrier transfer machines and cranes. They are non-redirective, meaning they do not deflect vehicles that impact the side back into the roadway. The energy of the impacting vehicle accelerates the water in the barrels vertically and laterally consuming that energy in work done on the water. Additionally,