Impact attenuator
Encyclopedia
An impact attenuator, also known as a crash cushion or crash attenuator or cowboy cushions, is a device intended to reduce the damage done to structures, vehicles, and motorists resulting from a motor vehicle
collision. Impact attenuators are designed to absorb the errant vehicle's kinetic energy
and/or redirect the vehicle away from the hazard, and from roadway machinery or workers.
Impact attenuators are usually placed in front of fixed structures near freeways, such as gore point
s, Jersey barrier
introductions, or overpass
supports. Temporary versions can be used for road construction projects.
Impact attenuators can be categorized by the method used to dissipate kinetic energy:
Truck-mounted versions (TMA), similar to railcar buffer
s, can be deployed on vehicles that are prone to being struck from behind, such as snow plows and road construction or maintenance vehicles. When deploying a truck-mounted attenuator, the work zone supervisor must recall that the attenuator may not completely absorb the energy of the collision. The truck may be propelled forward by the crash. 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 into the workers or machinery it is meant to protect. This is especially important in mobile work zones, where the truck's parking brake may not be engaged.
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, impact attenuators are tested and classified according to AASHTO Manual for Assessing Safety Hardware (MASH), which superseded the Federal Highway Administration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.
A Fitch Barrier is a type of impact attenuator which is essentially a plastic barrel filled with sand or water, usually yellow colored with a black lid.
The "Fitch Highway Barrier System", invented by race car driver John Fitch
, comprises a series of such impact attenuators often found in a triangular arrangement at the tip of a guardrail
between a highway
and an exit lane (the area known as the gore
), along the most probable line of impact. The barriers in front contain the least sand, with each successive barrel containing more; so that when a vehicle collides with the barrels they shatter, the kinetic energy
is dissipated by scattering the sand, and the vehicle decelerates smoothly instead of violently striking a solid obstruction, reducing the risk of injury to the occupants.
Fitch barriers are widely popular due to their effectiveness, low cost, and ease of setup and repair or replacement. Since first being used in the late 1960s, it is estimated that they have saved as many as 17,000 lives.
Fitch has stated that the design was inspired by sand-filled fuel cans which he used to protect his tent from strafing
during World War II
.
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...
collision. Impact attenuators are designed to absorb the errant vehicle's kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
and/or redirect the vehicle away from the hazard, and from roadway machinery or workers.
Overview
Gore (road)
A gore, gore point, or gore zone is a triangular piece of land found where roads merge or split. When two roads merge, the area is sometimes referred to as a merge nose...
s, Jersey barrier
Jersey barrier
A Jersey barrier or Jersey wall is a modular concrete barrier employed to separate lanes of traffic. It is designed to both minimize vehicle damage in cases of incidental contact while still preventing crossover in the case of head-on accidents....
introductions, or overpass
Overpass
An overpass is a bridge, road, railway or similar structure that crosses over another road or railway...
supports. Temporary versions can be used for road construction projects.
Impact attenuators can be categorized by the method used to dissipate kinetic energy:
- Momentum transfer. Many early models used successive rows of= sand- or water filled barrels or modules. Momentum is transferred to the sand or water, reducing the speed of the impacting vehicle.
- Material deformation. Many newer attenuators use crushable materials 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.
Truck-mounted versions (TMA), similar to railcar buffer
Buffer (rail transport)
A buffer is a part of the buffers-and-chain coupling system used on the railway systems of many countries, among them most of those in Europe, for attaching railway vehicles to one another....
s, can be deployed on vehicles that are prone to being struck from behind, such as snow plows and road construction or maintenance vehicles. When deploying a truck-mounted attenuator, the work zone supervisor must recall that the attenuator may not completely absorb the energy of the collision. The truck may be propelled forward by the crash. 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 into the workers or machinery it is meant to protect. This is especially important in mobile work zones, where the truck's parking brake may not be engaged.
In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, impact attenuators are tested and classified according to AASHTO Manual for Assessing Safety Hardware (MASH), which superseded the Federal Highway Administration
Federal Highway Administration
The Federal Highway Administration is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two "programs," the Federal-aid Highway Program and the Federal Lands Highway Program...
NCHRP Report 350. Classification is based on the maximum speed of a vehicle during a collision for which the attenuator is designed.
Fitch Barrier
The "Fitch Highway Barrier System", invented by race car driver John Fitch
John Fitch (driver)
John Cooper Fitch is a racecar driver born in Indianapolis, Indiana. He was the first American to race automobiles successfully in Europe in the postwar era...
, comprises a series of such impact attenuators often found in a triangular arrangement at the tip of a guardrail
Guardrail
Guardrail can refer to*Guard rails installed on road sides for automobile safety*RC-12 Guardrail, a U.S. Army intelligence-gathering aircraft based on the C-12 Huron*Roof edge protection, rails installed on roofs to protect construction and roofing workers...
between a highway
Highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
and an exit lane (the area known as the gore
Gore (road)
A gore, gore point, or gore zone is a triangular piece of land found where roads merge or split. When two roads merge, the area is sometimes referred to as a merge nose...
), along the most probable line of impact. The barriers in front contain the least sand, with each successive barrel containing more; so that when a vehicle collides with the barrels they shatter, the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
is dissipated by scattering the sand, and the vehicle decelerates smoothly instead of violently striking a solid obstruction, reducing the risk of injury to the occupants.
Fitch barriers are widely popular due to their effectiveness, low cost, and ease of setup and repair or replacement. Since first being used in the late 1960s, it is estimated that they have saved as many as 17,000 lives.
Fitch has stated that the design was inspired by sand-filled fuel cans which he used to protect his tent from strafing
Strafing
Strafing is the practice of attacking ground targets from low-flying aircraft using aircraft-mounted automatic weapons. This means, that although ground attack using automatic weapons fire is very often accompanied with bombing or rocket fire, the term "strafing" does not specifically include the...
during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.