Roadside ditches or swales are integral features of highways and are critical for control of stormwater runoff. These features can be obstacles to errant motorists that leave the roadway. The Fatality Analysis Reporting System (FARS) indicates that over the past 15 years more than 1,000 fatalities annually are attributed to ditches. The AASHTO Roadside Design Guide provides some guidance on preferred configurations for ditches. However, this guidance is based on the results of limited testing and simulations conducted in the 1970s. Additionally, there is variation in the practices across the states for designing and maintaining ditches—and for many miles of roads, the ditches are a remnant of much older design standards. Limited right-of-way often dictates the configuration of ditches and in many cases the preferred configurations are not practical. Enclosed drainage systems are expensive and may result in additional requirements for treatment and discharge of the runoff. Other drainage elements such as culvert ends, inlets, headwalls, and holding basins may themselves become roadside obstacles. Installing a barrier to shield a ditch reduces the available clear zone, is not cost-effective in many cases, and presents maintenance and operational issues. The need exists to reduce the number and severity of crashes involving roadside ditches. By identifying factors involved in crash events and evaluating the dynamics of vehicles interacting with ditch elements, countermeasures can be developed and implemented to mitigate these crashes.
The objective of this research was to develop guidelines for cost-effective treatments of roadside ditches and appurtenances in order to reduce the severity of ditch crashes.
STATUS: Research is complete, and publication of final report is pending.