Roadside guardrails make our roads safer, but the ends of these guardrails need protection, or end treatments, to prevent them from becoming roadside hazards of their own. Recent court cases involving some end treatments have focused media and public attention on the potential hazards associated with these special features of guardrails.
The ends of guardrails are designed to absorb the energy of vehicles that crash head-on into them. There are many different designs to achieve this goal, but two “extruder-type” end treatments have received most of the recent attention (Figure 1). These two extruders are mounted onto the end of common W-shaped steel guardrails and are then pushed along the guardrail by the vehicle during a crash. The moving extruder flattens the guardrail and bends it back on itself, a process that absorbs energy and slows down the vehicle. The posts supporting the guardrail also break away, absorbing additional energy.
Figure 1: Extruder type guide rail end treatment
Extruder end treatments are designed for primarily frontal vehicle impacts, and therefore are most commonly installed on straight roadways. Extruder end treatments can fail to operate properly under some conditions: if they are improperly installed, if their design is defective, or if vehicles strike them too obliquely. These failures can cause three outcomes:
- The extruder head bends back with the steel beam and allows the vehicle to get behind the guardrail before slowing the vehicle;
- The extruder head drops to the ground and the vehicle may then ride up and over the extruder and guardrail; or,
- The extruder head “locks” on the steel beam and penetrates (spears) the vehicle.
All guardrail end treatments must pass a series of crash tests before being approved for roadside installation. Early tests conformed to a standard issued in 1981but most currently installed end treatments meet a revised, more comprehensive standard issued in 1992. Since 2009 end treatments are required to meet a third standard that is similar to the second standard. The crash tests in these standards attempt to capture typical impact conditions, and include passenger cars and larger pickups striking the end treatments at speeds up to 60 mph (100 km/h) at a maximum angle of 25 degrees. The test vehicles are driven along a straight path and only the front of the vehicle strikes the end treatment. In practical terms, this means that there are real-world crash conditions (e.g., angled or side impacts) that fall outside of the standard test conditions and may result in failures of the end treatments.
Cases involving the potential failure of a guardrail end treatment need to be properly investigated. Key steps in the investigation process include i) examining the vehicle, guardrail and end treatment, ii) determining if the installed device was approved, iii) assessing the appropriateness of the end treatment for the specific location, and iv) documenting how the end treatment was installed and maintained. All of these factors then need to be interpreted in the context of the specific vehicle and collision dynamics. A roadside safety engineer can help evaluate how well a guardrail end treatment functioned and whether a properly functioning device could have affected the collision outcome.
Trinity ET-Plus and its predecessor the ET-2000 are the extruders involved in the recent court cases
National Cooperative Highway Research Program: NCHRP-230 “Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances”, TRB, 1981
NCHRP-350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features”, TRB, 1993
“Manual for Assessing Safety Hardware (MASH)”, American Association of State Highway Officials (AASHTO), 2009