NCHRP Report 794, Median Cross-Section Design for Rural Divided Highways,
presents guidelines for designing typical cross-sections for medians on new and existing rural freeways and divided highways. The report examines the interrelationships between median width, median slope, and the use of median barrier on crash risk and severity. The report should be useful to designers and safety analysts, particularly those responsible for agency standards.
The AASHTO Policy on Geometric Design of Highways and Streets contains general median width and median side-slope design guidance that has remained unchanged for many years. However, changes have occurred in the vehicle fleet, travel speeds, and traffic volumes that warrant further examination of this guidance. Concern with rollover crashes has caused many state departments of transportation (DOTs) to flatten their depressed medians. More recently, highly visible cross-median crashes have caused many state DOTs to increase their use of median barrier beyond the recommendations in the in the AASHTO Roadside Design Guide. There is speculation that flatter medians have contributed to the cross-median problem, but the data do not present a clear picture. Installation of median barrier reduces the number of cross-median crashes but increases the number of fixed-object crashes. Understanding how different median cross-section designs influence different types of crashes is vital in making safe and cost-effective decisions for state design standards and for project design.
In NCHRP Project 22-21, MRIGlobal, in association with the Pennsylvania State University, updated the survey of state practice developed in NCHRP Project 17-14, -14, including the types of barrier being installed and the policies for their installation. The research team also compiled information on typical median cross-sections for new construction and reconstruction projects. Based on the literature, the researchers identified design, traffic, and human factors that influence median and roadside safety. The research team then collected field data to assess the safety and cost-effectiveness of various median cross-section designs. Simulations of median encroachments were also made to evaluate the contributions of the various factors to cross-median crashes.