ROADSIDE GRADING GUIDANCE - PHASE I

Provisions for the design of roadside foreslopes are not readily available. As a result, engineering judgment is often employed. Unfortunately, this can lead to inconsistent designs, where, inevitably, some designs will be too costly and other designs will be too dangerous. Therefore guidance has been compiled to lend consistency to the design of these foreslopes while maintaining the most economical and safe design. This guidance was prepared after conducting a benefit-cost analysis using the Roadside Safety Analysis Program (RSAP). A large test matrix was developed in an attempt to simulate the most possible scenarios, leaving interpolation to a minimum. However, before the analysis could be run, the severity indices associated with foreslopes needed to be updated to accurately reflect vehicle damage and injury levels caused during an encroachment occurring at an average impact speed. Current indices are overestimated because they were based on a survey given out to highway safety officials who were most likely biased toward high-speed accidents. More data is being collected and will be added to the results of this report in phase two. To update the severity indices, accident data from the State of Ohio was analyzed using a program called Global Mapper, which allowed the user to measure topographical features, such as foreslopes, heights, and offsets. A method to account for underreported accidents on flat slopes is presented as well. Finally, equations for determining accident cost as a function of the traffic volume are given in conjunction with examples that demonstrate the use of these equations

Correlation Between Crash Severity and Embankment Geometry

The severity of a roadside feature is often based on survey responses and has tended to emphasize extreme crash events, thereby overestimating the average severity of a particular feature. In this study, severity was related to embankment geometry by examining real-world accident data over a 7-year period. This was done by correlating the number of severe and fatal accidents to the exposure of particular slope geometries. Slope geometry was described by slope steepness and fill height, and its exposure was described by traffic volume and total unshielded mileage. Severity was adjusted for posted speed limits as well. The Roadside Safety Analysis Program (RSAP) was calibrated such that the distribution of severe injury and fatal accidents accurately reflected real-world data. Using this calibrated version of RSAP, the new severity indexes were studied and equations were created to correlate severity index to functional class, fill height, slope steepness, and posted speed limit. The local highway classification provided the highest severity, and the default severity used in RSAP was increased to accommodate this finding. Freeways, rural arterials, and urban arterials experienced reduced severity indexes relative to default values used in RSAP

Cost-Benefit Analysis of Crash Cushion Systems

Crash cushions vary in geometry and cost. In this study, crash cushions were categorized in three different categories: redirecting with repair costs greater than $1,000 (RGM), redirecting with repair costs less than$1,000 (RLM), and nonredirecting sacrificial (NRS). Typically, RGM systems are less expensive initially, but life-cycle costs are high. RLM systems typically reciprocate this trend. NRS crash cushions (e.g., sand barrels) are generally less expensive but require total replacement after a crash has occurred, which may be impractical at high-traffic volume locations. Due to limited funding, there is often a need to identify the most cost-effective crash cushion category for highway scenarios with different roadway, traffic, and roadside characteristics. This study was commissioned to determine benefit-cost ratios for each crash cushion category in a wide range of roadway and roadside characteristics using the probability-based encroachment tool, Roadside Safety Analysis Program. Only RGM and RLM systems were cost effective for freeways and divided rural arterials, but all three categories competed against the unprotected condition on undivided rural arterials and local roads