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120 CONCLUSIONS This research marks one of the first major run-off-road crash data studies where both urban and rural data were studied simultaneously. General observation from this research include observed differences between urban and rural areas. It was observed that when all of the characteristics of the roadway are equal except the area type (i.e, urban and rural), more ROR crashes can be expected on urban roadways. On the other hand, curvature and grade increase ROR crashes in rural areas, but have little influence in urban areas. Similarly, shielding influences the crash severity outcome more in rural areas than in urban areas. The AASHTO Roadside Design Guide may consider these observed differences in crash frequency between areas types in future guidance development. The approach taken in this research will allow for these CMFs and future CMFs developed to accompany these models to be used in both the HSM approach and the RSAPv3 encroachment probability model which is referenced in the AASHTO Roadside Design Guide. In short, RSAPv3 will continue to be the go-to tool for the detailed analysis of roadside scenarios and the development of roadside policy. The new ROR SPF of the HSM will be the go-to tool for corridor assessment of roadside issues. A draft HSM chapter has been appended to this report as Appendix A. Draft HSM Chapter. A summary of the recommendations for improvements to RSAPv3 has also been developed and appended to this report as Appendix B. Modifications to RSAPv3. While these two tools have the same basic objective (i.e., quantifying the safety of particular design decisions) they operate in fundamentally different ways and are used at different points in the design process. The HSM has rigorous statistically based prediction methods based on observable crash data, which gives users confidence in the results. The disadvantage is that the CMFs in the HSM related to the roadside are very general and tend to have limited applications such that detailed design questions cannot be addressed with confidence. For this reason, the HSM is a good tool for corridor analysis and overall scoping of projects (e.g., how many ROR crashes might be avoided if we flatten the side slope, flatten a horizontal curve or widen the shoulders?) but a poor choice for detailed roadside-specific design (e.g., should the median barrier be placed at the center of the median or should two parallel runs be used at the edge of shoulder, should we use a flexible barrier system or a rigid concrete barrier?). On the other hand, RSAPv3 is based on a theory of crash causation and can be used to examine very detailed design issues. It would also be unwieldy to use RSAPv3 to perform a corridor analysis of a highway network or a long section of highway since all the detailed roadside geometry must be defined and modeled. Both approaches and both tools have a place in the design of highways and roadsides. In summary, two basic approaches have evolved for quantifying the frequency and severity of roadside crashes â the encroachment-based approach represented by RSAPv3 and the crash-based approach represented by the HSM. Both are important tools for highway design practitioners that enable them to explicitly quantify the economic and safety benefits of different design alternatives. Both tools provide the engineer with important information about how effective different design alternatives might be.
