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5 CHAPTER 1 Introduction Background (Antonucci et al., 2004). The desired result would be a set of Crash Modification Factors (CMFs) that specify the ratio of In 1997, the American Association of State and Highway the expected crash frequency after and before the implemen- Transportation Officials (AASHTO) Standing Committee for tation of a treatment. It is expected that the CMFs developed Highway Safety along with Federal Highway Administration would augment the CMFs currently in the Highway Safety (FHWA), National Highway Traffic Safety Administration Manual. (NHTSA), and the Transportation Research Board (TRB) Committee on Transportation Safety Management convened a meeting of national experts in the highway safety area to Study Objectives and Overview develop a Strategic Highway Safety Plan. This plan focuses on The objective of this project was to evaluate the safety effec- 22 highway safety challenges or emphasis areas that have an tiveness for selected strategies identified in NCHRP Report 500, impact on highway safety. To advance the implementation of Volume 12. The intent is to develop reliable CMFs. At a min- countermeasures to reduce accidents and injuries, NCHRP imum, for CMFs to be reliable they must meet the following Project 17-18(3) began the development of a series of imple- criteria: mentation guides which were subsequently published in the form of several volumes of NCHRP Report 500. Each guide addresses one of the 22 emphasis areas, and includes an intro- 1. CMFs should be methodologically and statistically duction to the problem, a list of objectives for improving safety valid. Many existing CMFs are derived from before-after in that emphasis area, and strategies for each objective. Each analysis of actual countermeasure implementation. Indeed, strategy is designated as: proven, tried, or experimental. such before-after studies, as opposed to cross-sectional/ Many of the strategies discussed in these guides have not regression-type analysis, will produce the best CMF esti- been rigorously evaluated. FHWA has initiated a Low Cost mates, but only if conducted properly to account for the Safety Improvements Pooled Funds study to evaluate some of regression to the mean effects at sites selected for treatment these strategies. The first two phases have been completed and because of unusually high accident frequencies. Unfortu- included the evaluation of the following strategies: (1) cen- nately, much of the available knowledge on CMFs may ter TWLTLs for two-lane roads, (2) higher retro-reflectivity be tainted by this problem because this selection bias is quite sheeting for STOP signs, (3) pavement markings noting `stop prevalent. Other methodological problems that have ahead,' (4) flashing beacons at stop controlled intersections, affected the reliability of currently available CMFs include: (5) the trade-off between lane and shoulder width given a Failure to properly separate out the safety effects of other pavement width, (6) advance street name signs, (7) curve changes (e.g., traffic volumes, the impacts of other mea- delineation, and (8) offset left-turn lanes at signalized inter- sures, crash reporting, underlying trends across time). sections. These evaluations were conducted using before-after Sample sizes that are too small--large numbers of data from locations where the safety improvements have been sites with the same combination of applied counter- made and a reference group of untreated locations. measures are needed for a valid analysis. For some treat- As a follow-up to these FHWA evaluations, NCHRP initiated ments that are expected to affect a low proportion of NCHRP Project 17-35. The focus of this project is to select the total crashes at a site (e.g., pedestrian treatments), and evaluate strategies from NCHRP Report 500, Volume 12: hundreds of locations may be necessary along with many A Guide for Reducing Collisions at Signalized Intersections years of crash data.

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6 Use of comparison groups that are unsuitable for a In Task 1, based on a critical review of published studies for variety of reasons, including the fact that sites may have each treatment/strategy, the research team assigned a level been affected by the treatment. of predictive certainty for each available CMF. Four levels Incorrect interpretation of accuracy of estimates or pre- of predictive certainty were defined: High, Medium-High, senting results without statements of accuracy. Low-Medium, and Low. The literature review also covered For many treatments, there may be different effects at knowledge about ongoing or planned research, which was different sites, so a single CMF that is typically estimated based on a review of several research-in-progress databases, is often not applicable. discussions with other highway safety researchers, and con- 2. The CMFs should represent the different crash categories versations with research sponsors such as FHWA and the that reflect the impact of the improvement. Crash cate- Insurance Institute for Highway Safety (IIHS). Details about gories might include total crashes, severe injury crashes, the literature review are presented in Chapter 2. property damage only crashes, and specific crash types In Task 2, web-based surveys were conducted using a (such as rear end and angle). tool called Zoomerang. The survey was sent to two listservs, 3. The variability in CMFs should be stated. The best estimate contacts in all 50 States, and several local agencies. The re- of the CMFs, along with some technique that reflects their spondents were asked to indicate if they had implemented a variability (such as ranges, confidence intervals, standard particular strategy/treatment and the approximate number of deviation, or some other technique) should be presented. installations for each strategy/treatment. In addition, they This will facilitate not only the application of the CMF were also asked to rate the importance of knowing the CMF but also the amalgamation with CMF results from other for each treatment. These ratings along with the information evaluation studies. about the installations were used to develop a short list of 4. The CMF should reflect the savings in "total harm" that treatments for further consideration. To get further informa- the treatment provides. Many treatments affect both crash tion about the availability and suitability of the data for the frequency and crash severity, some just severity, and some short-listed strategies, selected agencies that responded to the tradeoff crashes of different severities (e.g., traffic signaliza- original web-based survey were contacted by telephone. Further tion can decrease more-severe angle crashes but increase description of Task 2 can be found in Chapter 3. less-severe rear-end crashes). CMFs must capture changes In Task 3, the research team developed a recommended in severity as well as frequency in order to measure "harm and prioritized list of strategies to be evaluated to provide the savings." best use of available funds, and a work plan for evaluating these The identification and development of CMFs that meet strategies. Task 4 involved a meeting with the NCHRP panel most of these requirements involved a study effort with the to discuss the work plan and develop the list of strategies to following tasks: be evaluated in Task 5. Further description about Tasks 3 and 4 can be found in Chapter 4. Task 1--Literature Review Task 5 involved an evaluation of the strategies. The results Task 2--Conduct a Survey of State and Local Agencies from this evaluation are provided in Chapter 5. Chapter 6 Task 3--Develop a Work Plan provides a summary page showing the recommended CMFs Task 4--Meet with the Panel for each treatment that was evaluated in this study. Chapter 7 Task 5--Collect Data and Conduct Evaluation provides some general conclusions and directions for further Task 6--Develop a Final Report research.