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107 C H A P T E R 7 The conclusions of the research are as follows: 1. For traversable medians, median widths were found to have partially offsetting effects on median-related crashes. CMCs and NCMCs decrease with increasing median width, while rollover crashes increase. The net result of these effects is typically a slight increase in total median-related crashes with increasing median width. Even so, wider medians generally have a positive safety effect, because the severity of CMCs is much greater than for rollover crashes. Predictive models or SPFs for all median-related crashes and specific crash types are presented in Tables 4-6 and 4-7. The severity distributions for specific crash types are shown in Table 6-1. 2. For traversable medians, the crash analysis results indi- cated flatter slopes generally lead to more CMCs and fewer rollover crashes. This is a concern for use of flatter slopes, because CMCs are substantially more severe than rollover crashes. However, the results of vehicle dynamics simu- lation analysis do not indicate any trend toward CMCs becoming more likely for slopes flatter than 1V:8H. 3. Vehicle dynamics simulation indicates that there is an identifiable dividing line between combinations of median width and median slope for which the most likely result of a vehicle encroachment is a CMC and those combina- tions for which the most likely result is a vehicle rollover. The dividing line or boundary is generally a median width in the range of 15 to 17 m (50 to 55 ft) for median slopes steeper than 1V:8H and 18 m (60 ft) for median slopes of 1V:8H or flatter. This result is shown in Figure 5-24. 4. Vehicle dynamics simulation results suggest that a median with flatter slopes in the center is less conducive to CMCs than a median with uniform slopes that meet at the center of the median. 5. CMFs for flexible, semi-rigid, and rigid barriers have been developed in a before-after evaluation using the EB method. These CMFs are presented in Table 6-3, with supporting documentation in Tables 4-30 and 4-31. 6. A benefit-cost analysis based on the CMFs in Table 6-3 indicates that each of the three barrier typesâflexible, semi-rigid, and rigidâis cost-effective when applied in appropriate situations. Flexible median barriers are typi- cally used continuously for extended sections of median. Flexible median barriers may be cost-effective even at lower traffic volumes than suggested in the AASHTO median barrier warrants. Semi-rigid barriers are typically used in shorter lengths than flexible barriers and are placed at spe- cific obstacles. Rigid barriers are less cost-effective in rural divided highway medians and are primarily applicable to continuous sections of median than are too narrow to accommodate the deflection of a flexible barrier. The following recommendations were developed in the research: 1. The AASHTO Green Book (1) recommends 1V:6H slopes within medians, with 1V:4H slopes considered adequate in some cases. Based on the research results, it is recom- mended that the Green Book be changed to recommend 1V:8H slopes within medians, with 1V:6H slopes consid- ered adequate in some cases. It also is recommended that slopes flatter than 1V:8H be considered near the center of the median, where practical. 2. It is recommended that the median barrier warrants in the AASHTO Roadside Design Guide (2) be changed to indi- cate that barrier be considered for median widths up to 18 m (60 ft) where the median slope is less than 1V:8H. 3. It is recommended that the CMFs for median barrier instal- lation shown in Table 6-3 be considered for inclusion in the AASHTO Highway Safety Manual (63), potentially in conjunction with the SPFs for median-related crashes Conclusions and Recommendations
108 presented in Tables 4-6 and 4-7. These CMFs are suitable for planning of roadside design policies that would be applied over many sites, or to analyses conducted with a combination of an SPF for median-related crashes and the application of the EB method. However, these CMFs are probably not a suitable tool for application to individual sites without use of an SPF and the EB method, because individual sites are unlikely to have experienced a suffi- cient number of CMCs to make application of the CMFs accurate. Median barrier installation is more appropriately determined from policies based on median widths and traf- fic volumes, like those included in AASHTO policy, than on the analysis of crash data for individual sites. 4. The vehicle dynamics simulation results in Figure 5-25 show a broad range of simulated bumper heights for vehicles traversing a median, particularly after those vehi- cles have passed the swale and are traversing the upslope toward the opposing roadway. This has potential implica- tions for barrier placement and barrier mounting height. The results in Figure 5-25 suggest that small passenger car bumper traces remain relatively constant near the median swale, so that placement of a median barrier near the swale may be effective for this vehicle class. However, the range of bumper heights is larger for small SUVs after crossing the swale point so, for SUVs, the effectiveness of barriers placed on the upslope is a potential concern. No simu- lations of barrier impacts have been performed in this research; such simulations, especially for barriers located on the upslope beyond the median swale, should be con- sidered in NCHRP Project 22-22.
