other when collided with to absorb the impact. Their main benefit is in their reusability; these attenuators can automatically return to their original position after a crash. Truck mounted attenuators Truck mounted attenuators (TMA), 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). In some countries, such as Sweden, road safety authorities operate dedicated TMA vehicles registered as emergency vehicles, able to be dispatched to blockages or emergencies. Regulation 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impact attenuators are tested and classified according to AASHTO Manual for Assessing Safety Hardware (MASH), which superseded the Federal Highway Administration NCHRP Report 350.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. See also Crumple zone Shock absorber Traffic barrier SAFER barrier References Road safety Road infrastructure