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35 CHAPTER 7 Conclusions The importance of reliable estimates of the effectiveness The objective of this project was to evaluate the safety impact of safety improvements has become more apparent as safety of selected strategies from NCHRP Report 500, Volume 12: decisions have become more data-driven and safety analysis A Guide for Reducing Crashes at Signalized Intersections and has become more sophisticated. Specifically, SAFETEA-LU develop reliable CMFs. CMFs were developed for the following was signed into law in 2005, creating a positive agenda for five treatments at signalized intersections: increased safety on our highways by nearly doubling the available funds for infrastructure safety. With the increased · Installation of Dynamic Signal Warning Flashers, funding, SAFETEA-LU also required strategic highway safety · Convert Signalized Intersection to Roundabout, planning (i.e., data-driven decision-making), which increased · Increase Clearance Interval, the need for information to quantify the effects of safety · Change Left-Turn Phasing from Permissive to Protected- strategies. Permissive, and The CMF is one important piece of information to sup- · Install Flashing Yellow Arrow. port a data-driven decision-making process. CMFs indicate the expected effectiveness of a given strategy and allow Based on the evaluations, CMFs and measures of uncertainty agencies to compare the relative benefits of multiple treat- are provided in this report. Each strategy and evaluation is ments. Programs such as the Highway Safety Improvement also described with respect to methodology, sample size, Program (HSIP) necessitate the use of CMFs in the priori- and general applicability. While CMFs for five strategies were tization of funding for safety improvements. Additionally, developed as part of this effort, there still remain several several new safety tools such as the Highway Safety Manual strategies in the NCHRP Report 500 series without quality and SafetyAnalyst incorporate CMFs in their safety analysis CMFs. To help identify priority strategies for future research, process. the research team also conducted a survey of practitioners to Several large separate efforts have been undertaken to determine the CMFs that are of greatest need to them. Based develop reliable estimates of the safety effectiveness of improve- on the priority ratings from practitioners and an assessment ments. As a result of NCHRP Project 17-18(3), a series of of the current status of CMF knowledge of specific treat- implementation guides was developed and subsequently ments, the following list identifies treatments at signalized published as part of the NCHRP Report 500 series. Each intersections that may be considered as high priority for volume of the series addresses one of the 22 emphasis areas future research. The critical component for future evaluations from the National AASHTO Strategic Highway Safety Plan, and is the agencies who are currently installing these strategies includes an introduction to the problem, a list of objectives and the related data. Rigorous evaluations are only possible for improving safety in that emphasis area, and strategies for when accurate and reliable data are available for the strategy each objective. Expected effectiveness (i.e., a CMF) was pro- of interest. vided for some of the strategies, but many strategies did not have (and still do not have) an associated CMF. In some cases, · Install left-turn lane along with changes to left-turn phasing; the existing information related to the effectiveness was not · Coordinate signals along corridors; based on a rigorous evaluation. · Provide split phasing;
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36 · Lengthen existing left-turn lanes; In addition to the development of quality CMFs, it is · Delineate turn path inside an intersection; necessary for the practitioner to apply the CMFs appropriately. · Utilize crossing guards for school children; Users are encouraged to consider the quality and applicability · Replace standard signal heads with 12 signal heads; of CMFs when selecting a CMF for use in the decision-making · Restrict turning movements; process. Users are also encouraged to consider the measures of · Install pedestrian countdown signals; and uncertainty (e.g., standard error or standard deviation) associ- · Install additional signal heads. ated with a given CMF.