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1 S u m m a r y The AASHTO Policy on Geometric Design of Highways and Streets, commonly known as the Green Book, contains general median width and median side slope design guidance that has remained unchanged for many years. Crashes where a vehicle crosses the median and continues into the opposing lanes are often very severe. Concern with rollover crashes has caused many state highway agencies to flatten their depressed medians. More recently, Chapter 6 of the AASHTO Roadside Design Guide was revised in 2006 to increase the range of situations in which use of median barrier is considered. There is speculation that flatter medians have contributed to cross-median crashes, but the data do not present a clear pic- ture. Understanding how different median cross-section designs influence different types of median-related crashes is vital in making safe and cost-effective decisions for state design standards. The objective of this research was to develop improved guidelines for designing median typical cross sections (i.e., width, slope, and barrier) on new and existing rural divided high- ways. The scope of the research focused on the medians of rural freeways (i.e., facilities with full access control), but rural nonfreeway facilities also were considered. The research included review of current literature on median design guidelines and studies of the safety of various median designs and median barrier effectiveness. A survey on state practices in median design also was conducted. Crash analysis and simulation of vehicle incursions into medians of various design were conducted. Consideration of the results of both the crash analysis and the simulation of vehicle incursions provided a complete picture of the effects of each design variable on overall median safety and cost-effectiveness. Median Width The crash analysis results for fatalities and injuries on rural four-lane freeways generally indi- cate that cross-median crashes (CMCs)âcrashes that involve a vehicle crossing the median, entering opposing traffic, and colliding with an opposing-direction vehicleâdecrease with wider medians, while rollover crashes generally increase with wider medians. These two effects are of almost equal magnitude, but in opposite directions. The crash analysis shows a monotonic relationship between crashes and median width, suggesting that CMCs would keep decreasing and rollover crashes would keep increasing continuously as the median width increases. The results of the vehicle dynamics simulation show a more subtle interpretation of this relationship. Specifically, the vehicle dynamics simulation results indicate that, at a median width in the range from 15 to 18 m (50 to 60 ft), there is a boundary at which the probability of a CMC becomes less than the probability of a rollover crash. This suggests that when the lower severity of rollover crashes is taken into account, there are diminishing returns in continuing to make the median wider. Median Cross-Section Design for Rural Divided Highways
2Median Slope The crash analysis indicates that the median slope ratio also has opposing effects for CMC and rollover crashes, but that these opposing effects for median slopes are opposite to the effects for median width. Crash prediction models for rural four-lane freeways show that flatter slopes are associated with more CMCs and fewer rollover crashes. The models indicate that flatter slopes on rural four-lane freeways also are associated with fewer fixed-object crashes. The vehicle dynamics simulation analysis again provides a more complete understanding of the subtleties of median slope effects, as it did for median width effects. In this case, the vehicle dynamics simulation results show an interaction between median slope and median width not evident in the crash analysis results. For median slopes in the range from 1V:4H to 1V:7H, the boundary between medians for which CMCs are most prevalent, and those for which rollover crashes are most prevalent, falls in the median width range from 15 to 17 m (50 to 55 ft). For median slopes of 1V:8H or flatter, that boundary falls at 18 m (60 ft). Thus, the vehicle dynamics simulation results indicate that the concerns about high-severity CMCs are of greatest concern for median widths less than 18 m (60 ft) and for median slopes steeper than 1V:8H. Furthermore, the vehicle dynamics simulation results suggest that the likelihood of CMCs does not continue increasing as the median slope becomes flatter than 1V:8H. Median Barriers Crash prediction models developed for traversable and barrier medians can be used to estimate the safety differences between these median types with various geometric character- istics and barrier types. In addition, a before/after evaluation of median barrier installation estimated crash modification factors (CMFs) for flexible, semi-rigid, and rigid median barri- ers. The analysis results show that flexible barriers (i.e., cables), semi-rigid barriers (i.e., steel guardrail), and rigid barriers (i.e., concrete) can all be cost-effective in reducing crashes under appropriate conditions. A benefit-cost analysis shows that all of these barrier types can be cost-effective under appropriate conditions in reducing severe CMCs, while increasing less severe crashes of other types. Flexible median barriers may be cost-effective even at lower traffic volumes than shown in current AASHTO median barrier warrants.
