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, 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 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 John Fitch after the
1955 24 Hours of Le Mans race when his co-driver,
Pierre Levegh rear-ended
Austin-Healey driver
Lance Macklin 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
the worst accident in racing history. Fitch stated he was inspired by sand-filled fuel cans which he used to protect his tent from
strafing during
World War II. 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 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 and approximately $400 million per year in property damage and medical expenses.
Non-gating in
San Bruno, California 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. while others require minimal repairs.
Truck mounted attenuators Truck mounted attenuators (
TMA), also known as an
impact protection vehicle (
IPV) are deployed on vehicles that are prone to being struck from behind, such as
snow plows and road maintenance vehicles, most commonly on
motorways. The TMA may be stationary or move at a slow speed. Work zone regulations often specify a minimum buffer distance between the TMA and the work area, and a minimum mass for the truck, to minimize the chances that the TMA will be pushed forward by a crash into the workers or machinery. This is especially important in mobile work zones where the TMA's
parking brake may not be engaged. 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. TMA vehicles are better suited for protecting emergency or construction workers on roadways than non-specialized vehicles, such as firetrucks. TMA vehicles have several benefits over firetrucks, such as lower purchase and repair cost, improved firefighting capacity (because firetrucks are available for use and won't be damaged), and enhanced safety for the occupants of the impacting vehicles. This is because TMA vehicles are designed to absorb the energy of the impact, while firetrucks are too rigid and can cause severe injuries or death. ==Regulation==