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1 S U M M A R Y Factors Contributing to Median Encroachments and Cross-Median Crashes Research was conducted to investigate the factors that contribute to median-related crashes and to identify design treatments and countermeasures that can be applied to improve median safety on divided highways. The research used a combination of interdisciplinary field studies of locations with high frequencies of median-related crashes and systemwide crash data analy- sis including sites with the full range of median-related crash frequencies. The interdisciplinary field studies included both engineering and human factors specialists who assessed the factors that contributed to median-related crashes at 47 divided highway sites with high median- related crash frequencies in four states. Wherever practical, the results of the interdisciplinary field studies were confirmed through crash data analyses. Based on interdisciplinary field studies of sites in four states with high median-related crash frequencies, the following factors were found to contribute to the occurrence of median-related crashes on divided highways: ⢠On-ramps, ⢠Off-ramps, ⢠Closely spaced on- and off-ramps, ⢠Sharp horizontal curves, ⢠Steep grades, ⢠Bridges, ⢠At-grade intersections, and ⢠Wet and snow-covered pavement conditions. These factors were found to contribute to median-related crashes both individually and in combination. A separate analysis of crash data for rural freeways in Washington confirmed that the following factors are overrepresented in median-related crashes: ⢠On-ramps, ⢠Off-ramps, ⢠Sharp horizontal curves (particularly curves with radii less than 3,000 feet), ⢠Steep grades (particularly grades of 4 percent or more, including both upgrades and downgrades), and ⢠Wet and snow-covered pavement conditions. Other potential contributing factors, and combinations of contributing factors, could not be verified as contributing to median-related crashes either because of limited sample sizes of sites and crashes or because of lack of systemwide data. Although no separate
2confirmation could be developed for some factors, the interdisciplinary field studies, by themselves, provide evidence that all of the factors listed above contribute to median-related crashes. The findings summarized above indicate that improvements to the contributing factors listed have the potential to reduce the frequency of median-related crashes and can supple- ment traditional median safety programs that focus on reducing the consequences of leaving the roadway and encroaching on the median. The research confirmed the importance of the traditional approach to improving median safety, which involves design improvements to reduce the consequences of median encroach- ments. The following design improvements are recommended to implement this approach to improving median safety: ⢠Remove, relocate, or use breakaway design for fixed objects in medians; ⢠Provide barrier to shield objects in medians; ⢠Provide wide medians; ⢠Provide continuous median barrier; ⢠Flatten median slopes; ⢠Provide U-shaped (rather than V-shaped) median cross sections; and ⢠Provide barrier to shield steep slopes in median. The research also found that median safety can be improved by design treatments and countermeasures to reduce the likelihood of median encroachments (i.e., using design treat- ments and countermeasures to make it less likely that motorists will run off the roadway into the median). Design treatments recommended to reduce the likelihood of median encroachments include the following: ⢠Provide wider median shoulders, ⢠Minimize the use of sharp horizontal curves with radii less than 3,000 feet, ⢠Minimize use of steep grades of 4 percent or more, ⢠Increase separation between on- and off-ramps, ⢠Minimize left-hand exits, ⢠Improve design of merge and diverge areas by lengthening speed-change lanes, ⢠Simplify design of weaving areas, and ⢠Increase decision sight-distance to on-ramps. High-cost treatments, such a realigning curves or grades, may be impractical for existing roadways and may be applicable primarily in design of new construction projects. The following countermeasures are recommended to reduce the likelihood of median encroachments: ⢠Provide edgeline or shoulder rumple strips; ⢠Improve/restore superelevation at horizontal curves; ⢠Provide high-friction pavement surfaces; ⢠Improve road surface or cross-slope for better drainage; ⢠Improve visibility and provide better advance warning for on-ramps; ⢠Improve visibility and provide better advance warning for curves and grades; ⢠Improve delineation; ⢠Provide transverse pavement markings; ⢠Provide weather-activated speed signs;
3 ⢠Provide static signs warning of weather conditions (e.g., bridge freezes before road surface); ⢠Apply sand or other materials to improve road surface friction during winter storms; ⢠Apply chemical de-icing or anti-icing as a location-specific treatment; ⢠Install snow fences; and ⢠Raise the state of preparedness for winter maintenance. The contractorâs final report included four appendixes: ⢠Appendix A, Survey Questionnaire; ⢠Appendix B, Predictive Models for Median-Related Crashes from NCHRP Project 22-21; ⢠Appendix C, Site-by-Site Summary of Interdisciplinary Field Review Sites; and ⢠Appendix D, Guidelines for Reducing the Frequency and Severity of Median-Related Crashes on Divided Highways. Appendixes A through C of the contractorâs final report are not published herein but are available from the NCHRP. Appendix D presents recommended guidelines for reducing the consequences and like- lihood of median-related crashes on divided highways. The guidelines address the applica- tion of each design treatment or countermeasure together with known information on the effectiveness of each design treatment or countermeasure.
