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