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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 6 CHAPTER 2. RESEARCH APPROACH RESEARCH METHODOLOGY FOR NCHRP PROJECT 17-76 The research was originally proposed to be conducted within seven tasks. Each task listed is followed by the objectives of that task: ï· Task 1. Identify Current Knowledge and Practices: The objective of this task was to review the literature related to speed and associated relationships. In addition, the factors or variables used in current speed setting practices were identified. ï· Task 2. Identify Sources for Data: The objectives of this task were to identify sources for the following data: speed, traffic, roadway, enforcement practices, crash data, and any other additional critical data as identified in Task 1. Because of concerns with the amount of effort that could be needed with respect to obtaining data, during the kickoff meeting, the panel expressed their support for the research team not collecting new data with respect to enforcement. The panel also expressed a preference that the research team consider traffic mode split (e.g., between vehicles and bicyclists) in the analyses while also recognizing the challenges with obtaining such data. ï· Task 3. Develop Research Study Approach: The objective of this task was to develop the research study approach proposed for use in Phase II. In Task 3, the development was accompanied by a beta test of the approach to identify where refinements were needed. ï· Task 4. Develop Interim Report and Phase II Work Plan: The objectives of this task were to develop the work plan to be carried out in Phase II of the research project; develop the interim report that would summarize the results of Tasks 1 through 3; submit the Phase I interim report and the Phase II work plan; and participate in a panel meeting. ï· Task 5. Conduct Phase II Studies: The objective of this task was to conduct the approved work plan regarding the identification of variables that influence operating speed and safety on a road or street with a focus on the effects of speed term differences. ï· Task 6. Develop and Test Guidance Materials: The objective of this task was to develop the guidelines and then test the materials at a workshop. ï· Task 7. Prepare Final Documents: The objective of this task was to prepare the final documents. PHASE I EFFORTS During the kickoff meeting for NCHRP Project 17-76, the panel identified three conditions where scientific-based methods/guides for setting speed limits were desired: ï· Rural high-speed highways, where there is political pressure to increase the posted speed limit. ï· Rural-to-city transition zones, where there is pressure to lower speed. ï· Urban/suburban city streets, where there is pressure to adopt a uniform low speed (such as 25 mph).
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 7 The National Association of City Transportation Officials (NACTO) and Vision Zero are contributing to this discussion and using speed-related pedestrian/bike crash survivability to justify uniformly low posted speeds. In accordance with the direction set by the panel, the research team focused the Phase I literature review efforts on these three areas. Other Phase I efforts were grouped into the following broad areas: ï· Conduct project management. ï· Identify existing procedures for setting the posted speed limit. ï· Review literature with an emphasis on identifying known relationships among operating speed, safety, and roadway characteristics. ï· Identify potential operating speed datasets. ï· Obtain samples of speed data and review the quality and quantity of the speed datasets with respect to their usability for NCHRP Project 17-76. ï· Review other techniques for selecting posted speed limits. The research team met with the panel at the end of Phase I. In addition to approving the research teamâs proposed Phase II activities, the panel set the following directions during the Phase I panel meeting: ï· The SLS-Tool should include the proposed expanded functional classification scheme as discussed in NCHRP Research Report 855 (3). ï· Two studies were recently completed regarding rural-to-city transition zones (NCHRP Synthesis 412  in 2011 and NCHRP Report 737  in 2012). With the recent studies on transition zones, NCHRP Project 17-76 should focus on corridors that are not transitioning to another functional classification. ï· The decision support tool needs to be transparent (no black box). PHASE II EFFORTS Following the Phase I panel meeting, the following research efforts were conducted in Phase II: ï· Build and analyze a roadway characteristic, posted speed limit, operating speed, and crash database for Austin, Texas. ï· Build and analyze a roadway characteristic, posted speed limit, and crash database for Washtenaw County, Michigan. ï· Develop an SLS-Procedure that will result in an unbiased and objective method for identifying a suggested speed limit value. Identify appropriate decision rules for use in the procedure. ï· Automate the SLS-Procedure into an SLS-Tool using a spreadsheet as the base format. ï· Conduct selected technology transfer activities. ï· Document the efforts within the project.