Risk Assessment of Various Median Treatments of Rural Interstates

This report presents results of a comprehensive study that evaluates the safety impacts of alternative median treatments on rural freeways. Extensive data were obtained from Indiana, Colorado, Illinois, Iowa, Missouri, New York, Ohio, Oregon, Washington, and Wisconsin. The studied median treatments include AASHTO-recommended treatments: depressed medians, depressed medians with high and low tensioned cable barriers, and flush medians with concrete barriers. Other treatments used in the freeway design were also studied: medians with berms, sloped medians, and depressed medians with berms. Negative binomial models were developed to predict the frequency of crashes in three categories: single vehicle (SV), multiple vehicles same direction (MVSD), and multiple vehicles opposite directions (MVOD). Logit models were developed to split the frequency of crashes into two severity categories: fatal/injury crashes and property damage crashes. The obtained results support the findings of previous research that narrowing a freeway median increases the frequency of cross-median crashes. Furthermore, the frequency of MVSD crashes and SV crashes increase on segments with berm median treatments. Sloped median treatments are also attributed with an increase in the SV crashes. The majority of the developed frequency equations have a variable that represents the road curvature; it was primarily the average horizontal curvature. Freeway segments with medians with reduced width tend to experience more severe crashes than segments with wide medians. Curvature, predominately the presence of horizontal curves, also had a significant negative effect on the severity of crashes. Obtained equations can be used by designers and planners to predict the frequency and severity of crashes for alternative median solutions on rural freeways in the states that had contributed the research data. The results of this study help designers and planners select better median treatments on modernized rural freeways that often involve adding traffic lanes, narrowing medians, and installing barriers. The developed equations can be used as part of economic analysis of safety, construction, and maintenance costs.