Characteristics of Crash Cushions

Crash cushions are impact attenuators developed to prevent errant vehicles from impacting fixed obstacles. The crash cushion should either decelerate the vehicle to a safe stop, such as in a head-on hit, or redirect it safely away from the obstacle, in the case of a side hit. Crash cushions are typically used where fixed objects cannot be removed, relocated, converted to a breakaway design, or shielded by a longitudinal barrier. Examples of application sites include exit ramp gores where a bridge rail end or bridge pier presents a hazard, and the ends of longitudinal barriers. Most crash cushions are patented systems developed and tested by the manufacturer, who can also provide design charts for selection of appropriate designs.

Most crash cushions perform their function by the principle of kinetic energy absorption or transfer of momentum. In the first case, energy is absorbed by materials or devices that crush or plastically deform, or by hydraulic devices. A rigid backup support is required for such compression crash cushions. In the second case, the momentum of the vehicle is transferred to an expendable mass, such as containers filled with sand. No rigid backup support is needed for such “inertial” barriers. Some crash cushions use a combination of these principles.

Table 6.9 provides a list of some of the most common crash cushions in use today, and their applicable test levels. A description of these systems follows.

The Advanced Dynamic Impact Extension Module (ADIEMII) is a proprietary terminal designed to shield the end of a concrete safety-shape barrier. It consists of a 30 ft-long (9.1-m) carrier beam or concrete base structure onto which 10 interlocking perlite con­crete crushable modules are mounted. Energy is dissipated by crushing the modules. Perlite is an expanded inert mineral that, when substituted for coarse aggregate in a concrete mix, results in an extremely lightweight and crushable material. Strength lev­els in the perlite concrete are closely controlled to ensure that it falls within acceptable levels compatible with acceptable vehicle deceleration limits.

The Brakemaster 350 is a proprietary design used primarily as a terminal for W-beam median barriers or as a crash cushion to shield narrow obstacles. If used to terminate a concrete median barrier, a suitable transition is required between the device and the con­crete. It may also be used to shield the end of a roadside barrier but may not be cost — effective. The manufacturer recommends use in low-frequency impact areas. This terminal consists of an anchor assembly with posts embedded in the ground, a cable/brake assembly, and W-beam panels supported by steel diaphragms that slide backward in end-on hits. When impacted end-on, the W-beam panels telescope and the cable/brake assembly absorbs most of the energy through frictional resistance. The anchor assembly also provides sufficient anchorage to redirect side-impacting vehicles. A paved instal­lation pad is not required.

The Crash Cushion Attenuating Terminal (CAT) is a proprietary, nonflared attenuator commonly used to terminate W-beam median barriers and as a crash cushion to shield narrow fixed objects. A transition design is required for the latter case. It is sometimes used to shield a W-beam roadside barrier, but in that case, a cable anchor is required at the downstream. The CAT can redirect vehicles striking its face from one side or both

sides. The CAT functions as a three-stage system, utilizing energy-absorbing beam elements, breakaway wood posts, and a cable anchorage system. The beam element is a slotted W-beam that telescopes during impact. Shearing of the steel rail between the slots dissipates energy.

The bullnose guardrail system provides a nonproprietary means for shielding an object in the median of a divided highway by constructing a thrie-beam guardrail envelope around the end. Several such designs using W-beam guardrails have been constructed by highway agencies in the past, but these did not meet the criteria of NCHRP Report 350. However, a design that has met TL-3 consists of slotted thrie — beam panels mounted on breakaway posts near the nose, followed by standard thrie-beam posts and blocks toward the back of the system. Rail tension is developed through cable anchors and struts. A set of steel retention cables is mounted on the back of the thrie-beam nose to contain vehicles in the event of rail fracture. The leading edge of the bullnose attenuator should be located a minimum distance of 62 ft (19 m) in advance of the shielded object.

The ABSORB 350 is a proprietary, nonredirective, crash cushion primarily designed to shield the ends of the Quickchange® median barrier. This is a narrow cushion that may also be used to shield ends of concrete barriers or narrow fixed objects. The system is comprised of multiple, water-filled, energy-absorbing elements; a nosepiece assembly; and a transition/attachment assembly. Three length configura­tions are available.

QuadGuard refers to a family of proprietary devices with similar design and per­formance characteristics. The design consists of several types of energy-absorbing cartridges supported by a framework of steel diaphragms and corrugated steel fender panels. A concrete pad and rigid backup are required. Crash energy is dissipated by telescoping rearward and crushing the cartridges. The devices meet TL-2 or TL-3, as indicated in Table 6.9. Many parts of the various systems are interchangeable. The standard QuadGuard is a bidirectional device used as an end treatment for a concrete barrier or narrow fixed object. The cartridges must be replaced after an impact. The QuadGuard Wide is similar but can be used to shield wider objects. The QuadGuard LMC (Low-Maintenance Cartridge) is a self-restoring, bidirectional end treatment used at locations where a moderately high frequency of impacts is anticipated. It can be used to shield rigid barriers or fixed objects and is available in two widths. Energy­absorbing components are elastomeric cylinders that are reusable after most design impacts. The QuadGuard Elite is a self-restoring, bidirectional end treatment for loca­tions with high-impact frequency. It can be used to shield rigid barriers or fixed objects and is available in two widths. The energy-absorbing components are high — density polyethylene cylinders that are reusable after most design impacts.

The Trinity Attenuating Crash Cushion (TRACC) is a proprietary system. Components include a pair of guidance tracks, an impact “sled,” intermediate steel frames, and W-beam fender panels. A concrete pad and rigid backup are required. The sled, or impact face, contains a hardened steel blade that absorbs energy by cutting metal plates on the sides of the guidance tracks as it is forced backward. The interme­diate frames that support the fender panels are free to slide backward on an end impact, but lock onto the guidance tracks on a side impact to redirect the vehicle.

The Reusable Energy-Absorbing Crash Terminal (REACT 350) is a proprietary system comprising single row of 0.9-m-diameter (3-ft), high-density, polyethylene cylinders atop steel skid rails; a restraining cable system consisting of two heavy steel wire rope assemblies along each side; a front and rear anchorage system; transition hardware; and a backup assembly. A nine-cylinder array meets TL-3 and a four-cylinder design meets TL-2. The system may be used on either a concrete or an asphalt surface if properly anchored. The polyethylene cylinders absorb energy as they slide rearward on the steel railing, and are self-restoring in many cases. The steel cables redirect vehicles in side impacts. A wider REACT that can be used to shield fixed objects up to 5 ft (1.5 m) was tested successfully to TL-3. This design consists of two parallel columns of 2-ft-diameter (0.6-m) cylinders attached to steel diaphragms mounted on an anchored monorail, which provides redirection for side impacts.

The narrow Connecticut impact attenuation system (NCIAS) is a nonproprietary, bidirectional crash cushion that consists of eight steel cylinders in a single row with two anchored wire tension cables along each side. The cylinders, which are 3 ft (0.9 m) in diameter and 4 ft (1.2 m) high, crush to absorb energy. The tension cables keep the cylinders in place and provide redirection for side impacts. The last four cylinders are reinforced with pipe stiffeners and retainers to help redirect vehicles hitting close to the rear. The NCIAS is recommended for use where shielding of narrow objects is needed and reverse-direction impacts are unlikely.

Sand-filled plastic barrels, sometimes called inertial crash cushions or inertial barriers, are used in both temporary and permanent installations to shield the ends of longitudinal barriers or other fixed objects. The sand-filled barrels dissipate energy by transferring vehicle momentum to the variable masses of sand in the barrels that are hit. Standard module masses vary from 200 to 2100 lb (90 to 960 kg). A backup struc­ture or wall is not required because the force that a vehicle exerts on the individual modules is not transmitted through the cushion. Manufacturers have developed stan­dard arrays that can be used for specific types of fixed objects as well as design charts to analyze layouts. The barrels should be set as far from the traveled way as possible to minimize hits. However, the width of the last row should always be greater than the width of the shielded object. Moisture content of the loose sand should be 3 percent or less and clean sand should be used to minimize caking. A significant variation in the density of the sand could affect performance. Frozen sand reduces safety performance but mixing rock salt (5 to 25 percent by volume, depending on climate) with the sand generally prevents wet sand from freezing. The use of sacked sand to facilitate cleanup is not acceptable.

The gravel-bed attenuator provides a means to decelerate large trucks. Basically, the truck is slowed as the wheels move through a bed of gravel. It is typically used on truck escape ramps along descending highway grades where runaway vehicles present a problem. Crash cushions previously discussed are designed to stop or redirect passenger cars and pick­up trucks. They are not applicable to large vehicles, because considerable space is required to dissipate the energy. Detailed design guidelines for the gravel-bed attenuator are provided in the AASHTO publication A Policy on Geometric Design of Highways and Streets.

The Dragnet or chain-link fence vehicle attenuator is a proprietary device consist­ing of anchor posts, energy-absorbing reels of steel tape, and a net assembly to catch the vehicle. When impacted, the chain-link fence wraps around the front of the impacting vehicle and energy is absorbed as the metal tape is pulled through a series of rollers. The system may be repaired by replacing the steel tape in the casings and resetting the chain-link fence and cable. The Dragnet may be considered for locations where impacts are expected to be head-on and the results of vehicle penetration are severe, such as for temporary road and ramp closures, or in conjunction with a longi­tudinal barrier to shield the opening between twin bridges. It is designed to stop a 4500-lb (2000-kg) passenger car impacting head-on at 60 mi/h (100 km/h). It has also been used in series to stop large vehicles where space will not accommodate a gravel bed attenuator. Such a system safely stopped at 50,000-lb (22,700-kg) tractor-trailer impacting at 90° and 50 mi/h (80 km/h). Since the Dragnet deflects significantly, it can be used effectively only at locations where a sufficient clear area exists behind it. Because of the low deceleration rates resulting, very little damage is done to impacting vehicles and serious injuries to vehicle occupants are unlikely.

The Water Twister Vehicle Arresting System (VAS) is a proprietary system consisting of a chain-link restraining net connected to two energy-absorbing base units by nylon straps. As an impacting vehicle displaces the net, the straps turn shafts connected to turbine rotors inside the base units, which contain a water/ethylene glycol solution. Rotation of the turbine blades in the fluid dissipates energy. The base units are of sub­stantial size and may require shielding.