Date of Award
Master of Science
Civil, Construction, and Environmental Engineering
Peter T. Savolainen
Median-crossover crashes involve a vehicle departing the travel lane, traversing the median, and striking either a fixed object or an oncoming vehicle in the opposing direction of traffic. These types of crashes present the highest risk of fatal and severe injuries among all collision types on freeways. Median-crossover crashes are caused by a variety of factors, including driver distraction, impaired driving, mechanical failure, and loss of control. Median barriers are the primary countermeasure to reduce the opportunity for multi-vehicle cross-median crashes. Due to the lower installation costs, as compared to more rigid systems, high-tension median cable barrier has become a popular countermeasure for reducing such crashes. The Iowa Department of Transportation (Iowa DOT) began installing high-tension median cable barriers in 2003 and will have approximately 330 miles of such barrier installed on freeways statewide by the end of 2018. While anecdotal evidence suggests that cable barriers are functioning as desired, no in-depth analysis of performance or cost-effectiveness has been conducted to date. This project aims to determine the cost-effectiveness of median cable barrier systems based on crash cost savings resulting from reductions in fatal and injury crashes as compared to the costs associated with increased property-damage-only crashes, installation costs, and ongoing maintenance costs. An in-depth analysis of the frequency and severity of crashes occurring in the freeway median was conducted.
This research identified general trends in crash frequency and severity between the pre- and post-installation periods. Following an extensive manual review of crash narratives, a before-and-after study design showed the overall impact of the median cable barrier program was a 51.3% reduction in fatal crashes. The barrier program also showed an 80.4% increase in total crashes, which was driven by an increase in property-damage-only crashes of 118.1%. Installation and maintenance repair costs on the barrier were investigated and discussed. The design life of the median cable barrier was recommended to be 20 years with a benefit-cost ratio of 9.153. Geometric characteristics such as median width, shoulder width, and barrier offset were investigated, but no significant trends were identified. Future research as to the relationship between roadway geometric characteristics and median cable barrier effectiveness is recommended in order to plan for subsequent installations of median cable barrier in Iowa.
Ellen E. Nightingale
Nightingale, Ellen E., "Effectiveness of median cable barrier in Iowa" (2017). Graduate Theses and Dissertations. 15591.