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In the time it takes for the driver to yawn, a vehicle traveling at highway speeds can cross a highway median and strike opposing traffic head-on. Head-on crashes at highway speed are generally more severe than other types of highway crashes. In 2006, on the National Highway System alone, there were 821 median crossover crashes that resulted in fatalities.
Median barriers are longitudinal barriers most commonly used to separate opposing directions of traffic on a divided highway. While these systems may not reduce the frequency of crashes due to roadway departure, they can definitely help prevent a median crash from becoming a median crossover head-on collision.
Barrier Design and Placement Considerations
Barrier design and placement needs to effectively protect motorists traveling in opposing lanes, while also considering the safety of the occupants of the errant vehicle. Among the factors involved in selection of a barrier system are the types of vehicles using the roadway, the roadway geometry, and the potential severity of a median crossover crash. Standard barriers capable of redirecting passenger cars, light vans and trucks are considered cost effective for most situations. However, at locations with adverse geometrics, high traffic volumes and speeds, significant amounts of heavy truck traffic, or special environmental considerations, a higher performance median barrier may be more appropriate.
Pros and Cons of Barrier System Options
There are three basic categories of median barriers: rigid barrier systems, semi-rigid barrier systems, and flexible barrier systems.
Ease of maintenance and repair for these barrier systems is an important consideration. When several posts are hit during a single crash, the barrier system may then be vulnerable to subsequent crossovers until the damaged section is repaired. Some high tension systems are designed to reduce deflection and resist impact forces. These high tension systems can be easier to repair when posts are hit, and have the added advantage of being practical for use in narrower spaces.
For More Information
AASHTO-AGC-ARTBA Online Barrier Hardware Guide: http://aashtotf13.tamu.edu/
AASHTO's Technology Implementation Group Cable Median Barriers Website: http://tig.transportation.org/?siteid=57&pageid=2197
Charles F. McDevitt: "Basics of Concrete Barriers," Public Roads, Vol. 63 No. 5, March/April 2000: http://fhwicsint01.fhwa.dot.gov/publications/publicroads/00marapr/concrete.cfm
FHWA Crash Test Acceptance Letters for Longitudinal Barriers: http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/barriers/
Frequently Asked Questions: Barriers, Terminals, Transitions, Attenuators, and Bridge Railings: http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/qa_bttabr.cfm
Roadside Design Guide, AASHTO, 2006: https://bookstore.transportation.org/Item_details.aspx?id=148
Median Barriers: A Solution to Cross-Median Crashes, DVD, FHWA-SA-08-007.
Roadside Design: Steel Strong Post W-beam. A guidance memo was issued on May 17, 2010 on the height of guardrail for new installations. Guidance regarding existing guardrail will be developed in the next several months, in consultation with AASHTO’s Technical Committee on Roadside Safety.