Guidance on the application of cable median barrier: tradeoffs between crash frequency, crash severity, and agency costs

Abstract

Median-crossover crashes present the highest risk of fatality and severe injury among collision types on freeways. These crashes can be caused by a variety of factors, including drowsiness, driver distraction, impaired driving, and loss of control. The primary countermeasure to reduce the opportunity for such crashes is the installation of median barriers. The Michigan Department of Transportation (MDOT) began installing hightension cable median barriers in 2008, and has installed approximately 317 miles of cable median barrier on state freeways as of January 2014. Given the capital costs required for this installation program, a comprehensive before-after evaluation was conducted in order to ascertain the efficacy of cable barrier systems installed to date, and to develop guidelines to identify candidate locations for subsequent installations. Crash reports were reviewed to identify target median-related crashes and this manual review provided critical supplementary information not normally available from the standard fields on police crash report forms. Statistical analyses which accounted for regression-tothe-mean effects showed that fatal and incapacitating injury crashes were reduced by 33 percent after cable barrier installation. The analysis also showed the median cross-over crash rate was reduced by 86.8 percent and the rate of rollover crashes was reduced by 50.4 percent. In contrast, less severe crashes were found to increase by 155 percent after cable barrier installation. A detailed analysis of crashes involving a cable barrier strike found the barriers were 96.9 percent effective in preventing penetration through the barrier. Weather conditions, horizontal curvature, and offset of cable barrier from the roadway were also