Factors Contributing to Median Encroachments and Cross-Median Crashes (2014)

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4S E C T I O N 1 1.1 Background Median-related crashes on divided highways, which include cross-median crashes and other crashes in which one or more vehicles enter or encroach on the highway median, frequently result in high-severity injuries and fatalities. Most previous studies of contributory factors associated with cross-median crashes have typically focused on median width and average daily traffic (ADT). A few studies have looked at the influ- ence of geometry and cross-sectional elements. Although these studies have been helpful, they have not explored many other design and operational factors that may contribute to cross-median crash frequency or severity, such as presence of interchange ramps, spacing between interchange ramps, horizontal and vertical alignment, and pavement surface conditions. Median encroachments involve an unintentional entry into the median by a motorist. A median encroachment is part of the sequence of events for every median-related crash, including every cross-median crash. It has generally been assumed that most median encroachments result from driver inattention or fatigue, which leads to the driver drifting into the median and losing control. There is undoubtedly a sub- stantial proportion of median encroachments of this type, which explains the effectiveness of rumble strips on median shoulders. However, the available evidence suggests that many median encroachments may be initiated in a different way by vehicle–vehicle interactions on one roadway of a divided highway. In this situation, a motorist may lose control and run into the median (and, indeed, may cross the median and enter the opposing lanes) as a result of a multiple-vehicle collision or as a result of a maneuver to avoid a multiple-vehicle colli- sion. There has been very little research conducted on median encroachments that result from multiple-vehicle collisions or other vehicle–vehicle interactions. Similarly, most research on addressing median-related crashes has focused on countermeasures to reduce the fre- quency and severity of collisions that may occur after a vehi- cle has left the roadway and entered the median. Attention is needed on countermeasures that reduce the likelihood that vehicles will leave the roadway and enter the median in the first place. This research addresses the roadway factors that appear to initiate median encroachments by using the concept of driver workload. Roadway segments on divided highways have been classified as involving a range of driver workload levels from very high to very low. The research has sought evidence on how median encroachment and crash frequencies relate to the driver workload and how median- related crashes can be reduced by roadway improvements as well as median improvements. Relationships between factors related to cross-median crashes have been developed by MRIGlobal in NCHRP Proj- ect 22-21, “Median Cross-Section Design for Rural Divided Highways” (1). Variables that were found to affect median- related crash frequencies, in addition to ADT and median width, include roadside slopes, median shoulder width, pres- ence of horizontal curves, and presence of shoulder rumble strips. 1.2 Research Objectives and Scope The objectives of this research are to (1) identify design and operational factors and combinations of factors that con- tribute to the frequency of median encroachments and cross- median crashes and (2) identify potential countermeasures suitable for addressing these contributing factors. The scope of the research will address both the design of medians on divided highways and the design and opera- tion of adjacent roadways to minimize the occurrence of median encroachments that lead to cross-median crashes and to minimize the severity of crashes that occur. Both rural and urban divided highways will be considered in the research and the procedures developed will address both Introduction

5 1.4 Organization of This Report This report presents the results of the research on reducing the frequency and consequences of median encroachments and cross-median crashes. The remainder of this report is organized as follows. Section 2 describes the review of litera- ture on median encroachments, median-related crashes, crash countermeasures for improving median safety, road safety audits, and median design practice. Section 3 summarizes the survey of agency practice on median design and perceived median encroachment factors. Section 4 presents the results of interdisciplinary field reviews, including the contribut- ing factors identified. Section 5 presents the results of crash data analyses to investigate the contributing factors. Section 6 summarizes guidance on countermeasure implementation to address the contributing factors and reduce the frequency and severity of median-related crashes. Section 7 presents the con- clusions and recommendations of the research. Appendixes A through C of the contractor’s final report are not published herein but are available on the TRB website and can be found by searching for NCHRP Research Results Digest 390. Appendix D presents guidance on countermeasures to reduce median-related crashes on divided highways. Appen- dix D has been formatted so that it can be used as a stand-alone document, if highway agencies wish to use the guidelines in that form. controlled-access and non-controlled-access roadways (i.e., both freeways and divided nonfreeway facilities, also known as expressways). The primary focus of the research is on obtaining a better understanding of how median-related crashes are initiated and on providing guidelines that can be used by highway agencies to identify and select appropriate countermeasures to median-related crashes for specific road segments. 1.3 Research Approach The research approach has involved identification of more than 40 divided highway sections with high median-related crash frequencies. These divided highway sections were located in four states: California, Missouri, Ohio, and Wash- ington. An interdisciplinary team, including both a highway traffic engineer and a human factors specialist, reviewed the median-related crash reports and performed field visits to clas- sify the driver workload level for each site and identify factors that contribute to median-related crashes at each site. Broader crash data analysis was then conducted for the entire freeway network of one state to determine whether the factors identi- fied in the interdisciplinary field studies were, in fact, associated with higher likelihood of median-related crashes. The research findings were then used in the development of guidelines for implementing countermeasures to reduce the frequency of median-related crashes.