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

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65 S E C T I O N 7 The conclusions of the research are as follows: 1. Median encroachments are initiated both from loss of control by drivers of single vehicles and by vehicle–vehicle interactions that result in one or more vehicles leaving the roadway and entering the median. Review of crash data for sites with high frequencies of median-related crashes found that 73 percent of median-related crashes began with a single vehicle losing control, while 27 percent resulted from vehicle–vehicle interactions. 2. Review of crash data for sites with high frequencies of median-related crashes found that 55 percent of median- related crashes occurred under wet or snow-covered pave- ment conditions, while 45 percent of median-related crashes occurred under dry pavement conditions. Wet and snow-covered pavements appear to be overrepresented in median-related crashes, since the proportion of days with precipitation of 0.01 inches or more ranges from 16 to 39 percent for the four states studied (California, Missouri, Ohio, and Washington). 3. Based on interdisciplinary field studies for sites in Califor- nia, Missouri, Ohio, and Washington 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. 4. Countermeasures recommended to reduce the likelihood of median encroachments include the following: •• 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; •• 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. 5. Appendix D provides recommended guidelines for reduc- ing the consequences and likelihood of median-related crashes. The guidelines address the application of each design treatment or countermeasure together with known information on the effectiveness of each design treatment of countermeasure. Appendix D presents effectiveness measures for each design treatment or countermeasures whose effect on crash frequency or severity is known. Fur- ther research would be desirable to establish effectiveness measures for other design treatments and countermeasures. 6. A separate analysis of crash data for rural freeways in Washington confirmed that the following factors are over- represented in median-related crashes: •• On-ramps; •• Off-ramps; Conclusions and Recommendations

66 •• Sharp horizontal curves (particularly curves with radii of 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 confirmation could be devel- oped for some factors, the interdisciplinary field studies, by themselves, provide evidence that all of the factors listed in Conclusion 3 contribute to median-related crashes. 7. Conclusions 3 and 4 indicate that improvements to the con- tributing factors listed there have the potential to reduce the frequency of median-related crashes and can supplement traditional median safety programs that focus on reducing the consequences of leaving the roadway and encroaching on the median. 8. Crash data analysis found that as the radius of a horizontal curve decreases (i.e., as a curve gets sharper), the proportion of median-related crashes increases and the proportion of on-road crashes decreases. The following recommendations were developed in the research: 1. The traditional approach to improving median safety involves design improvements to reduce the consequences of median encroachments. The following design improve- ments 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. 2. The research confirms that median safety also can be improved by design treatments and countermeasures to reduce the likelihood of median encroachments (i.e., using design treatments and countermeasures to make it less likely that motorists will run off the roadway into the median). 3. 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 lengthen- ing 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 appli- cable primarily in design of new construction projects. 4. Countermeasures recommended to reduce the likelihood of median encroachments include the following: •• Provide edgeline or shoulder rumble 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; •• 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. 5. Appendix D provides recommended guidelines for reduc- ing the consequences and likelihood of median-related crashes. The guidelines address the application of each design treatment or countermeasure together with known information on the effectiveness of each design treatment or countermeasure. Appendix D presents effectiveness measures for each design treatment or countermeasure whose effect on crash frequency or severity is known. Fur- ther research would be desirable to establish effectiveness measures for other design treatments and countermeasures.