17.8 Attenuators (Crash Cushions)
17.8.1 Application
Crash cushions or impact attenuators are protective devices that prevent errant vehicles from impacting fixed objects. This is accomplished by gradually decelerating a vehicle to a safe stop for head-on impacts or redirecting a vehicle away from the fixed object for side impacts.
17.8.2 Crash Cushion Categories
Crash cushions are classified in one of three categories based on repair costs, lifecycle costs, labor and repair time after a head-on impact: sacrificial, reusable, and low maintenance. Crash cushions are classified in one of three categories based on repair costs, lifecycle costs, labor and repair time after a head-on impact: sacrificial, reusable, and low maintenance.
17.8.3 Sacrificia
These units typically require full replacement or substantial repairs either on-site or in a maintenance yard following an impact. These units have lower installation costs and should be considered for sites that typically experience less than one impact every 18 months. Waterfilled crash cushions are allowed for use in temporary work zones only.
17.8.3.1 Reusable
These units typically feature side fender panels, cartridges, or cylinders that absorb an errant vehicle’s energy during impact. Typically, the cartridges, or cylinders and a nose piece will have to be replaced after an impact. These units should be considered in locations that typically experience impacts in the range between one impact every 18 months and less than 3 impacts per year.
17.8.3.2 Low Maintenance
These units typically utilize plastic cylinders, or hydraulic mechanisms to absorb energy. These require some maintenance after an impact to ensure proper performance during the next impact. These have high initial cost and should typically be considered at locations where they will experience three (3) or more impacts per year.
For more detailed information on the installation of various types of attenuators, refer to
and
.
17.8.4 Location
Attenuators are ideally suited for use at locations where fixed objects cannot be moved, relocated, or made breakaway, and cannot be shielded by a longitudinal barrier. A common application of a crash cushion is in an exit ramp gore where a bridge rail end requires shielding. Crash cushions are also frequently used to shield bridge columns as well as roadside and median barrier terminals.
In temporary work zone applications using a parallel rigid barrier with narrow shoulder widths, if feasible, the barrier should be tapered slightly on the approach to allow additional shy distance before the placement of the crash cushion. This will reduce incidental impacts. The taper rate of the rigid barrier, and alignment and placement of the crash cushion should conform to
,
, and the manufacturer's specifications for the specific crash cushion type.
17.8.5 Standard Installations
There are numerous types of attenuators that are in common use. When more than one system is under consideration, the designer should carefully evaluate the structural, safety, and maintenance characteristics of each candidate system. Characteristics to be considered include the following:
- Category;
- What is being shielded;
- Space available for installation;
- Debris produced by impact; and
- Ease and cost of maintenance.