NOTE: These are sample definitions derived from other states. They are believed to be appropriate for use in California, but they may need modifications and/or clarifications.
Attenuator — An impact attenuator directly absorbs the energy of an impacting vehicle and reduces the force on a vehicle occupant to an acceptable level.
Barrier / Traffic Barrier — A device which provides a physical barrier through which a vehicle would not normally pass. Permanant barriers are designed to minimize harm to vehicle occupants; temporary barriers need also to protect construction and maintenance workers. Geometric and operational restrictions in work zones frequently preclude the use of the same design standards for barriers and terminals that apply to permanent systems.
Traffic barriers are designed so that a vehicle hitting the barrier is steered back onto the road. Concrete barriers redirect errant vehicles due to their shape; other barriers often redirect by designing supports so that they break off on impact, allowing the barrier to deform and push the vehicle back on track. All barriers either move on impact, as with concrete barriers, or deform, as with metal barriers. Accordingly, if used to protect work zones, all require a buffer zone between their placement and the area they are intended to protect.
BLON, Beginning Length of Need — The length of effective barrier needed upstream of the obstacle to adequately shield the obstacle. It includes the effective portion of the end treatment.
Clear Zone — The total roadside border area, starting at the edge of the traveled way, available for safe use by errant vehicles. This area may consist of a shoulder, a recoverable slope, and/or a traversable but nonrecoverable slope and a clear runout area. The minimum desired width is dependent upon the traffic volumes and speeds, and on the roadside geometry.
Crash Cushion / End Treatment
Directive End Treatment — An end treatment that can prevent an errant vehicle from impacting an obstacle by gradually decelerating the vehicle to a safe stop or by redirecting the vehicle away from the obstacle.
Nonredirective Crash Cushion — An end treatment that does not provide redirection capability for a side impact; a sand barrel array is an example. The vehicle does not normally reach the obstacle.
Crashworthy — A device that has been proven acceptable for use either through crash testing under specified conditions or through inservice performance.
Deflection — Barriers deflect when hit. To provent an impacting vehicle from contacting a shielded area, sufficient clear area must be present between the back of the barrier and the protected area. The amount needed will vary with the deflection distance of the barrier chosen. Also, if a barrier system deflects sufficiently, an impacting vehicle may lose contact with the ground, greatly increasing the chances of it going either over or under the barrier.
Design Speed — A selected speed used to determine the various geometric design features of the roadway, as well as the performance capability requirements for barrier systems.
Embankment slope — If an embankment is too steep and close to a barrier, there may not be adequate soil to provide lateral resistance during a crash into the barrier. One countermeasure is extra posts.
Federal Acceptance Letter — Results from testing by the Federal Highway Administration. This letter typically describes the device and tests performed, and reports test results and approval level.
For a list of all acceptance letters for barrier terminals and crash cushions, see http://safety.fhwa.dot.gov/roadway_dept/road_hardware/listing.cfm?code=cushions.
Longitudinal barrier results are available at http://safety.fhwa.dot.gov/roadway_dept/road_hardware/longbarriers.htm.
Bridge railing results are available at http://safety.fhwa.dot.gov/roadway_dept/road_hardware/longbarriers.htm.
Gating — Characteristic of an end treatment that allow a vehicle impacting the nose of the unit at an angle to pass through the device.
NonGating — Characteristic of an end treatment that has the capability of redirecting a side impacting vehicle essentially through its entire length and capturing the vehicle when impacted on the end at an angle of 15 or less.
Obstacle — Obstacles include both nontraversable terrain and fixed objects, and may be either manmade (such as critical embankments, ditches, bridge piers, signs, or headwalls) or natural (such as trees or boulders)
Operating Speed — The speed at which drivers are observed operating their vehicles during free flow conditions, generally taken as the 85th percentile speed.
Penetration — When a vehicle passes through an appurtenance either by overcoming its redirective resistance or by vaulting over or submarining under the appurtenance.
Post spacing / linking — Post spacing and/or linking of runs influence deflection.
Run Length — Short barrier runs do not redirect as well as longer runs. Short runs may serve as an attenuating structure, but may not redirect errant vehicles or shield fixed objects as desired.
Selfrestoring — Characteristic of an end treatment that almost returns to its original condition after impact.
Shy Distance — The distance from the edge of the traveled way, on the right hand side, within which a roadside object will be perceived as an immediate hazard by the typical driver to the extent that the driver will change the vehicle's placement or speed.
Transition — A section of barrier between two different barriers of different stiffness; most commonly, where a Wbeam barrier is connected to a bridge end post or other rigid object. It is only needed when the "softer" barrier is upstream of the "harder" barrier and should produce gradual stiffening so vehicular pocketing, snagging, or penetration is avoided.
Travel Lane — That portion of the pavement contained within the outer pavement marking lines (does not include shoulder).