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18 CHAPTER 3 Development of New AMFs through Analysis or Reanalysis of Crash Data This chapter provides a summary of each of the data analysis several AMFs. Many of the recommendations were based on efforts to produce or enhance AMFs. The narrative includes a a consensus that prior research was valid and applicable to the description of the treatment studied, the data used, the statisti- roadway class in question and thus that the AMFs from the cal methodology, and the results of each evaluation. More research were appropriate. Other recommendations required detailed descriptions of these efforts are included in appendices additional analysis to be conducted by one or more of the to this report. NCHRP project teams. Two of those analysis efforts were undertaken in this project. The first analysis effort focused on travel speed. While Introduction travel speed is known to be a critical factor in both crash Much of the Phase II effort involved new analysis or reanaly- frequency and crash severity, limited effort has been focused sis of crash and other safety data for the treatments identified by on producing a relationship that would allow prediction of the oversight panel and project team. Individual summaries are the crash-related effects of reducing average speeds by a given presented for the following four treatments: amount. If such a relationship could be developed, it could be used as an AMF for a wide spectrum of treatments with · Installation of a traffic signal at a rural intersection (new EB known effects on average speeds. For example, if the effects evaluation); on travel speed of changing a speed limit or installing a neigh- · Conversion of undivided four-lane road to three lanes borhood traffic calming device could be estimated, these including a two-way left-turn lane--a "road diet" (reanaly- effects could be converted into expected changes in crashes. sis of data from two previous studies); Based on the panel recommendation, a reanalysis of data · Increasing pavement friction on intersection approaches from a prior research study was conducted to confirm or (reanalysis of previous study data); and modify the results and to determine if the findings, which · Increasing pavement friction on roadway segments (reanaly- were based on the many non-U.S. studies, were applicable to sis of previous study data). U.S. roadways. A summary of this analysis is provided in this chapter. The EB analyses of the following four urban signalized- The second analysis effort was conducted by the suburban/ intersection treatments will be described in one section, since urban panel and involved examining existing studies of the the same reference group was used for all four: effects of median width on crashes. The panel was unable to come to a consensus on an AMF. At the panel's recommen- · Modification of left-turn signal phase (3 combinations), dation, the project team conducted additional analysis of this · Replacement of 8-in. signal head with 12-in. head, issue using HSIS data from California. Note that because me- · Replacement of single red signal head with two signal dian widths are not normally changed without changes in heads, and other critical roadway components (e.g., changes in number · Replacement of nighttime flashing operation with regular of lanes and/or shoulder width), a traditional EB before-after signal phasing. analysis was not possible. Instead, as described later in this chapter, cross-sectional regression analyses were conducted. Also, as noted in the Chapter 4 discussion, the analysis- Each of the summaries will provide information on the driven expert panels recommended additional analysis for treatment, the methodology, the data used, and the results of