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3 C H A P T E R 1 1.1 Background The AASHTO Policy on Geometric Design of Highways and Streets (1) contains general median width and median side slope design guidance that has remained unchanged for many years, not recognizing the dramatic changes that have occurred in vehicle fleet, travel speeds, and traffic volumes. Concern with rollover crashes has caused many state high- way agencies to flatten their depressed medians. Recently, Chapter 6 of the AASHTO Roadside Design Guide (2) has been revised to increase the range of situations in which use of median barrier is considered. There is speculation that flatter medians have contributed to the cross-median prob- lem, but the data do not present a clear picture. Certainly, 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 project design. NCHRP Project 17-14, Improved Guidelines for Median Safety, attempted to develop guidelines for using median bar- rier and selecting median widths and slopes. Unfortunately, collection of data needed for Project 17-14 proved to be very expensive, and the data limitations hampered the strength of the recommendations. To avoid some of the obstacles that Project 17-14 faced, Project 22-21, Median Cross-Section Design for Rural Divided Highways, focused on typical cross-section designs selected for a construction or reconstruction project rather than the exact cross-section design at a particular point. The typical cross-section designs are determined fairly early in the design process before adjustments are made to account for variations that occur along the alignment (e.g., horizontal and vertical curves, interchanges and intersections, and special drainage requirements). A related project, NCHRP Project 22-22, Placement of Traffic Barriers on Roadside and Median Slopes, will furnish additional guidance on the placement of median barriers. 1.2 Research Objectives and Scope The objective of this research was to develop improved guide- lines for designing median typical cross sections (i.e., width, slope, and barrier) on new and existing rural divided highways. Traffic volumes, clear zones, and drainage were considered. The guidelines are suitable for inclusion in the AASHTO Roadside Design Guide (2) and the AASHTO Policy on Geometric Design of Highways and Streets (1). The scope of the research addressed the design of medians on rural divided highways. The research focused on the medi- ans of rural freeways (i.e., facilities with full access control), but rural nonfreeway facilities also were considered. However, intersection areas on nonfreeway facilities were not considered, because such intersections are being addressed in a separate research effort in NCHRP Project 15-30, Median Intersection Design for Rural High-Speed Divided Highways. The issue of barrier end treatments at divided highway intersections was considered outside the scope of the research. The primary focus of the research was on documenting the safety performance of, and developing design guidelines for, traversable medians with no barrier and nontraversable medians with barriers because they appear to present the great- est design challenges and the greatest need for re-examination of current design policies. Nontraversable medians with no barriers (e.g., medians with trees or natural obstructions) were not considered in the research. 1.3 Organization of This Report This report presents an overview of the work conducted in the project. The subsequent chapters of this report are organized as follows. Chapter 2 describes the review of Introduction
4current median design guidelines, review of studies on the safety of median designs, state highway agency median safety research, and median barrier effectiveness evalua- tions. Chapter 3 summarizes the survey of state practice on median design and safety. Chapter 4 discusses the safety analysis of traversable medians and the safety analysis of medians with barrier. Chapter 5 details the simulation of vehicle encroachments on medians. Chapter 6 discusses a benefit-cost comparison of median design alternatives. Chapter 7 presents the guidelines for median cross-section design and the conclusions and recommendations of the research. Appendixes to this report are not published herein, but are available on the project webpage and can be found by searching the TRB website for NCHRP Report 794. The 2003 state-of-the- practice survey questionnaire used in NCHRP Project 17-14 is presented in Appendix A. The 2006 survey questionnaire used in this project is presented in Appendix B. Appendix C presents the logic used in each state to categorize median-related crashes. Appendix D discusses an approach to digital terrain mapping developed by MRIGlobalâs subcontractor, Pennsylvania Trans- portation Institute (PTI); this approach was used in the field to collect data on median side slopes and offset of median barriers. Appendix E details the benefit-cost analysis.
