Accident Modification Factors for Traffic Engineering and ITS Improvements (2008)

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68 AMFs are a key component of existing procedures used by state and local transportation agencies to choose and target safety treatments and of a series of new safety analysis tools being developed to make these safety decision processes even more effective and efficient. AMFs can be developed from an analysis of crash data, usually in a before-after evaluation, and from combinations of information from prior research studies. While there have been decades of effort aimed at developing credible AMFs, there have been surprisingly few developed that are of a quality that withstands critical scrutiny. However, in the past 10 years, there has been greatly in- creased emphasis on the development of credible AMFs. With this emphasis has come greatly increased funding, both in the NCHRP program funded by AASHTO and in FHWA’s re- search program. NCHRP funded the development of the NCHRP Report 500 guides, which have captured information on safety program effects from existing research literature in 20 different roadway, driver, and vehicle emphasis areas, in sup- port of the implementation of the AASHTO Strategic Highway Safety Plan. In response to knowledge needs cited in a group of the NCHRP Report 500 guides, FHWA is funding a continuing series of scientifically sound evaluations of low-cost treatments for roadways. NCHRP is also funding a series of projects aimed at development and publication of the Highway Safety Man- ual, including a detailed “current knowledge” section that cap- tures AMFs from published literature, the aforementioned project to develop crash prediction tools and AMFs for rural multilane highways, and the aforementioned project to de- velop similar tools and AMFs for suburban/urban arterials. The project effort described in this report is the final major AMF development effort recently funded by NCHRP. As shown in Table 18 (see Chapter 5) AMFs have been verified, modified, or developed here. When published this year, the Highway Safety Manual will provide a larger list of AMFs. However, even with this amount of concentrated effort, there remain a number of critical safety treatments, includ- ing many ITS treatments, for which credible AMFs do not exist. As described in Chapter 2, the research team developed and used a detailed procedure for ranking AMFs needing de- velopment. The procedure included not only an assessment of the current status of AMF knowledge (the level of predic- tive certainty) but also priority ratings by state DOT users, an estimate of crash-related harm possibly affected by the treat- ment, knowledge of ongoing or future research that might develop AMFs, and the current availability of needed evalua- tion data. Table 20 provides a listing of treatments that are considered to be high-priority targets for future development based on this ranking methodology. Clearly, others would be added if another ranking method was used. In summary, this project has verified, modified, or devel- oped 35 AMFs that are perceived to be of high or medium- high quality. These have been documented in formats that are usable by both practitioners and researchers. These AMFs are the primary project outputs. The project has also docu- mented a process that can be used with future analysis-driven expert panels, and notes from the detailed discussions of the two expert panels that were part of this effort can be provided to others. This material should be helpful in future efforts to develop or improve AMFs for treatments for which no AMF could be developed in this research. Finally, a procedure for ranking needed AMF research was developed and docu- mented—a procedure incorporating not only state DOT user and researcher opinions and knowledge of the quality of AMFs in the published literature, but also a method for esti- mating the crash-related harm that might be affected by each treatment. An approach combining these factors could be used in more global efforts to prioritize roadway safety re- search needs in general. C H A P T E R 6 Conclusions

69 High Priority Treatments (Gray shading indicates high ranking in user survey) Overall Ranking for Future Research* Intersection Treatments Channelize right turns MH Install dynamic advance warning flashers “Red Signal Ahead” MH Provide signal coordination MH Prohibit left turns MH Segment Treatments Add a travel lane MH Add shoulder rumble strips (two-lane/others) MH Add edgeline rumble strips MH Eliminate left turns at driveways MH Remove roadside obstacles (including urban) MH Flatten sideslopes MH Add advance curve warning signs/on pavement markings H Add midblock pedestrian signal MH Install raised crosswalk (non-intersection) MH * H = High, MH = medium high Table 20. High priority treatments needing AMF development in future research.