Problem: An alternative to traditional concrete and metal-beam barriers is needed because these structures can be expensive and difficult to install
Median crossover crashes often result in fatalities or severe injuries to occupants of the errant vehicle and the motorists in the opposing traffic lanes. State departments of transportation (DOT) are interested in reducing median crossover crashes through the use of median barriers. The concrete and metal beam barriers traditionally used to prevent these crashes, however, don't perform well on sloped terrain. In addition, concrete and metal beam barriers are expensive, and State and local agencies often lack the resources to rapidly deploy these technologies to areas where vehicles frequently cross over the adjacent medians.
Recent research shows that cable median barriers are more forgiving than traditional concrete and metal beam barriers and can be designed to be effective when installed on sloping terrain. Collision forces are deflected laterally thereby reducing the forces transmitted to vehicle occupants.
Solution: Promote the accelerated deployment of cable median barriers
Cable barriers are softer barriers and studies in Washington and North Carolina show a substantial reduction in fatal and injury crashes when compared to concrete and guardrail median barriers.
When design elements such as deflection and maximum slope requirements are met, cable can be an alternative to concrete and metal beam barriers. Some States are turning to cable median barriers in areas where there is sufficient median width and a high potential for crashes.
Although cable barriers have been used since the 1960s, it was not until the 1980s that some State DOTs started using a modified cable rail as a median barrier. Today, many States, including Arizona, Colorado, North Carolina, Ohio, Oklahoma, Oregon, South Carolina, Utah, and Washington State, are installing cable barriers in medians originally built without barriers. New data suggest that cable median barriers are an effective mechanism for preventing fatal and disabling crashes. In Washington, for example, annual crossmedian fatal crashes declined from 3.00 to 0.33 fatalities per 100-million miles of vehicle travel, while annual disabling accidents went from 3.60 to 1.76. The overall benefits of cable median barriers were calculated to be $420,000 per mile annually.
While cable median barriers have low installation costs, they can be to maintain due to the number of crashes that result in damage. Even though the cost of a crash is generally low, the system receives some damage from even slight hits, and needs to be repaired to provide optimum performance. Also, even low tension cable does not always go down on impact. It is more likely than high tension cable to go down after a hit.
In addition, when several posts are hit during a single crash, the cable barrier may then be vulnerable to crossovers until the damaged section is repaired. Several proprietary hightension cable designs are now available, however, that can withstand multiple hits.
Putting It in Perspective
Many States have collected data that demonstrate the significant impacts of crossmedian crashes:
Successful applications: Using cable barrier systems to meet local needs
The North Carolina and Oregon DOTs completed detailed in-service evaluation reports of cable barrier systems and found that the systems were nearly 100 percent effective in preventing deadly crossover crashes on freeways. Cable median barriers have been successful in South Carolina, where three-strand cable systems were installed in areas with multiple median crossover crashes. From August 2000 through July 2003, the South Carolina cable median system was hit 3,000 times, but only 15 vehicles penetrated the cables. In addition, several other State DOTs recently installed proprietary cable barrier systems with reported success.
Some motorcyclists have expressed concerns over cable barriers. Researchers in the United Kingdom, however, found little difference between crashes into cable median barriers and other barrier types. According to the data, most riders are separated from their motorcycles soon after leaving the pavement and are sliding on the ground by the time they reached the barrier. The data also did not show that cable barriers cause extraordinary injuries.
National Deployment Statement
The increased use of cable barriers in relatively wide medians where a barrier is warranted will decrease the number of severe cross-median crashes. Cable barriers are cost effective because they are relatively inexpensive compared to other types of barriers. They also perform better than other barriers when installed on the moderately sloping terrain common to many existing medians.
National Deployment Goal
The goal is for every State to review its median crossover crash history to identify locations where median barriers may benefit safety and to implement appropriate construction projects that use cable median barriers where appropriate.
National Deployment Status
Many State DOTs have independently assessed the extent of their median crossover problem. Most of these States also have installed a significant number of cable barriers, including both the lower-tension design and high-tension proprietary cable barriers.
Phase of Deployment
Phase IV-Deployment Activities
Guidelines for median barrier warrants, selection, and installation are contained in AASHTO's 2002 Roadside Design Guide. Information on crash-tested cable barriers can be found at http://safety.fhwa.dot.gov/report350hardware/ under Longitudinal Barriers, using the keywords, "Cable Barriers." To learn more about AASHTO-TIG's approved technologies, visit http://tig.transportation.org.
For More Information Contact:
Nick Artimovich, FHWA Office of Safety
Frank Julian, FHWA Resource Center
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