New cable barrier systems reduce costs – and save lives
By Ken Sweeney, MaineDOT Chief Engineer
How could someone possibly lose their lane discipline, cross a grass median and end up in the opposing lane? Crashes like this involve any of a variety of driver issues: unsafe speed, including during periods of heavy rain when the vehicle is susceptible to hydroplaning; alcohol; fatigue; and driver medical episodes. The driver may have tried to avoid a problem in their lane and, as a result, lost control. Whatever the reason, that fast-moving vehicle now traveling in the wrong direction toward high-speed traffic is a very big problem.
Interstate highways have median dividers to better separate opposing traffic flows. However, on occasion, cross median crashes do occur. This type of crash is not that frequent, but typically are among the worst crash scenarios because they are high-speed, head-on collisions. They are frequently devastating, often with fatal results.
To help prevent these crashes, there have been several roadside barrier systems developed, such as w-beam guardrail or what seems even better – concrete barriers. These rigid barrier systems have shortcomings, though, and vehicles tend to ricochet off them, bouncing them back into traffic causing additional hazards. Rigid barrier systems also are expensive, and w-beam guardrails, in particular, require rigorous maintenance and repairs.
The drawbacks of common barrier systems led MaineDOT to install median cable barriers on narrow sections of unprotected interstate median where cross median crashes are most likely to occur. Most often these crashes involve passenger vehicles, but in two recent Maine crashes, tractor-trailers were involved. Although the median cable barrier is not designed for heavy tractor-trailer loads, the system performed well in both cases, preventing the trucks from crossing into opposing traffic and therefore, probably saving lives.
Cable median barriers are more forgiving than traditional concrete and w-beam barriers and can be modified to be effective on sloping terrain. The posts are light, designed to bend and break, if needed, with the tensioned cable being the structural element that deflects the wayward vehicle and normally guides the car along the roadside. Cable barriers generally reduce collision forces and that translates to less severe injuries for the drivers and passengers along Maine roadways.
Installation costs for these cable systems are lower than standard w-beam systems, and repairs can be made more easily, quickly and inexpensively. The cable barriers also do not block blowing and drifting snow and do not contribute to snow build-up along side of roads.
Most importantly, the cable barriers greatly increase safety. No system will provide 100 percent protection due to the varying physical dynamics of a fast moving vehicle. Nevertheless, national studies, as well as case histories here in Maine, documented that these systems have saved lives.
In a Washington state study, data showed that annual cross median fatal crashes declined from 3.00 to 0.33 fatalities per 100-million miles of vehicle travel, while annual disabling accidents were reduced 50 percent. The overall benefits of cable median barriers were calculated to be $420,000 per mile annually.
North Carolina has installed hundreds of miles of cable median barriers since 1998 and estimates that between 1999 and 2005 more than 95 cross-median crashes were prevented, saving more than 145 lives. Wisconsin also demonstrated significant safety benefits, where a total of 181 miles of cable median barrier has been installed on state highways. The study’s 2009 update showed a 59 percent reduction in fatal and serious injury collisions.
While cable median barriers do have low installation costs, they can require more frequent maintenance due to the number of minor crashes that result in system damage. Minor crashes may become more frequent, since vehicles that may have stopped in the median after losing control in their lane now have the potential to strike the cable system. The cost of a crash generally is low, and is a worthy trade off when you consider the outcome of lower severity crashes versus a high-speed head-on collision.
Of course, the best way to avoid crash-producing situations in the first place is for drivers to avoid distractions, remain alert, and drive in a manner that keeps their vehicles under control and in their own lane.