![Crash Cushion System](https://upload.wikimedia.org/wikipedia/commons/4/48/Crash_Cushion_System.png)
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
A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on Track (rail transport), rails (such as trains or trams) and is used for the transportation of pe ...
collision
In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...
. Impact attenuators are designed to absorb the colliding vehicle's
kinetic energy
In physics, the kinetic energy of an object is the energy that 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 accele ...
.
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
highway
A highway is any public or private road or other public way on land. It is used for major roads, but also includes other public roads and public tracks. In some areas of the United States, it is used as an equivalent term to controlled-access ...
s, such as
gore points,
crash barrier
Traffic barriers (sometimes called Armco barriers,AK Steel (formerly Armco) genericized trademark also known in North America as guardrails or guard rails and in Britain as crash barriers) keep vehicles within their roadway and prevent them from ...
introductions, or
overpass
An overpass (called an overbridge or flyover in the United Kingdom and some other Commonwealth countries) is a bridge, road, railway or similar structure that crosses over another road or railway. An ''overpass'' and ''underpass'' together form ...
supports. Temporary versions may be used for road construction projects.
Operation
Impact attenuators are designed to absorb the colliding vehicle's
kinetic energy
In physics, the kinetic energy of an object is the energy that 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 accele ...
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
In particle physics, wave mechanics and optics, momentum transfer is the amount of momentum that one particle gives to another particle. It is also called the scattering vector as it describes the transfer of wavevector in wave mechanics.
In the s ...
. 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
Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geologi ...
deformation
Deformation can refer to:
* Deformation (engineering), changes in an object's shape or form due to the application of a force or forces.
** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies.
* Defo ...
. Many newer attenuators use crushable materials (like various kinds of foam) that create a
crumple zone
Crumple zones, crush zones, or crash zones are a structural safety feature used in vehicles, mainly in automobiles, to increase the time over which a change in velocity (and consequently momentum) occurs from the impact during a collision by a ...
, absorbing energy.
Others flatten a corrugated steel guard rail section, or split a steel box beam.
#
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of t ...
. 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
![Northern_Motorway1](https://upload.wikimedia.org/wikipedia/commons/8/8d/Northern_Motorway1.JPG)
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, this work is done over time which reduces the deceleration (negative acceleration) applied to the vehicle occupants. A smaller amount of energy is consumed in the work of crumpling the plastic containers. In cold climates, water-filled attenuators are either avoided, or have additives such as
magnesium chloride
Magnesium chloride is the family of inorganic compounds with the formula , where x can range from 0 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in natu ...
salt added to them to prevent freezing.
Fitch barriers
A Fitch barrier consists of sand-filled plastic barrels, usually yellow-colored with a black lid. The "Fitch Highway Barrier System" was invented by
race car driver
Auto racing (also known as car racing, motor racing, or automobile racing) is a motorsport involving the racing of automobiles for competition.
Auto racing has existed since the invention of the automobile. Races of various sorts were organise ...
John Fitch after the 1955 24 Hours of Le Mans race when his co-driver, Pierre Levegh rear-ended
Austin-Healey
Austin-Healey was a British sports car maker established in 1952 through a joint venture between the Austin division of the British Motor Corporation (BMC) and the Donald Healey Motor Company (Healey), a renowned automotive engineering and des ...
driver
Lance Macklin
Lance Noel Macklin (2 September 1919 – 29 August 2002) was a British racing driver from England. He participated in 15 Formula One World Championship Grands Prix, debuting on 18 May 1952. He was infamously involved in the 1955 Le Mans disaste ...
at high speed, launching his car through the air and into the spectator's area. The car burst into flames and took the lives of Pierre and 84 spectators in
one of the worst accidents in racing history. He stated he was inspired by sand-filled fuel cans which he used to protect his tent from
strafing
Strafing is the military practice of attacking ground targets from low-flying aircraft using aircraft-mounted automatic weapons.
Less commonly, the term is used by extension to describe high-speed firing runs by any land or naval craft such ...
during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
. Early prototypes were self funded and tested due to low initial support. As a proof of concept Fitch used liquor barrels filled with sand to create the necessary impact attenuation, then personally crashed reinforced vehicles into them while recording with a high speed camera to capture the rate of deceleration.
Fitch barriers are often found in a triangular arrangement at the end of a
guard rail
Guard rail, guardrails, or protective guarding, in general, are a boundary feature and may be a means to prevent or deter access to dangerous or off-limits areas while allowing light and visibility in a greater way than a fence. Common shapes ...
between a
highway
A highway is any public or private road or other public way on land. It is used for major roads, but also includes other public roads and public tracks. In some areas of the United States, it is used as an equivalent term to controlled-access ...
and an exit lane (the area known as the
gore
Gore may refer to:
Places Australia
* Gore, Queensland
* Gore Creek (New South Wales)
* Gore Island (Queensland)
Canada
* Gore, Nova Scotia, a rural community
* Gore, Quebec, a township municipality
* Gore Bay, Ontario, a township on Manitouli ...
), 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
In physics, the kinetic energy of an object is the energy that 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 accele ...
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 and approximately $400 million per year in property damage and medical expenses.
Non-gating
![Grafton_Gully_Turnoff_On_State_Highway_16](https://upload.wikimedia.org/wikipedia/commons/9/93/Grafton_Gully_Turnoff_On_State_Highway_16.jpg)
Non-gating impact attenuators arrest the motion of vehicles that impact head-on, whilst deflecting vehicles that impact the side of the barrier. They are anchored and more expensive, but can be used in tighter spaces.
Crash cushions
Crash cushions are constructed of multiple segments, which crumple into each 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
![Impact Attenuator In Auckland](https://upload.wikimedia.org/wikipedia/commons/9/98/Impact_Attenuator_In_Auckland.jpg)
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 plow
A snowplow (also snow plow, snowplough or snow plough) is a device intended for mounting on a vehicle, used for removing snow and ice from outdoor surfaces, typically those serving transportation purposes. Although this term is often used to re ...
s 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
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, impact attenuators are tested and classified according to
AASHTO Manual for Assessing Safety Hardware (MASH), which superseded the
Federal Highway Administration
The Federal Highway Administration (FHWA) 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 a ...
br>
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
Crumple zones, crush zones, or crash zones are a structural safety feature used in vehicles, mainly in automobiles, to increase the time over which a change in velocity (and consequently momentum) occurs from the impact during a collision by a ...
*
Shock absorber
A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Most sh ...
*
Traffic barrier
Traffic barriers (sometimes called Armco barriers,AK Steel (formerly Armco) genericized trademark also known in North America as guardrails or guard rails and in Britain as crash barriers) keep vehicles within their roadway and prevent them from ...
*
SAFER barrier
References
{{DEFAULTSORT:Impact Attenuator
Road safety
Road infrastructure