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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 4 CHAPTER 1. BACKGROUND Speed limit is the maximum speed a driver is legally permitted for a given roadway segment. Several types of speed limits exist, including statutory speed limit, posted speed limit (PSL), school zone speed limit, work zone speed limit, variable speed limit, and advisory speed (Figure 2 illustrates these different types of speed limits). Source: Federal Highway Administration Speed Limit Basics, page 1 (2). Figure 2. Examples of speed limits. A posted speed limit could be the same as the statutory speed set by the state legislature or could be an adjustment to the statutory speed limit determined using an engineering speed study. States establish statutory speed limits for specific types of roadsâsuch as freeways, rural highways, or urban streetsâwhich are applicable even if the speed limit sign is not posted. OBJECTIVE AND SCOPE National Cooperative Highway Research Program (NCHRP) Project 17-76 was tasked with identifying factors that influence a driverâs operating speed and then developing a Speed Limit Setting Procedure (SLS-Procedure) and automating that procedure with a spreadsheet- based Speed Limit Setting Tool (SLS-Tool). The SLS-Procedure and SLS-Tool are used to calculate the suggested speed limit for a segment. The goal of the SLS-Procedure and SLS-Tool is to produce an unbiased and objective suggested speed limit value. Traffic engineers can use the SLS-Procedure and the suggested speed limit generated by the SLS-Tool to communicate with the public or government officials to explain the methodology behind setting speed limits. Two publications were generated as part of NCHRP Project 17-76: ï· NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (1). ï· NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool (this document). ORGANIZATION OF WEB-ONLY DOCUMENT This report documents the research efforts and findings from NCHRP Project 17-76 in the following chapters and appendices: ï· Chapter 1. Background: This chapter introduces the objective of the research along with the chapters and appendices contained in this web-only final report. ï· Chapter 2. Research Approach: This chapter provides details on how the project was conducted.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 5 ï· Chapter 3. Findings: The findings are summarized in this chapter for each major effort within NCHRP Project 17-76. ï· Chapter 4. Conclusions, Recommendations, and Suggested Research: This chapter provides the conclusions generated from the research along with a list of suggested research needs and improvements to the SLS-Tool. ï· Appendix A. Relationship among Roadway Characteristics, Speed, and Safety for Urban/Suburban Streets: This appendix presents the findings reported in the research literature on the relationships among traffic crashes, operating speed, and roadway factors, including posted speed limit, for urban and suburban streets. ï· Appendix B. Relationship among Roadway Characteristics, Speed, and Safety for High-Speed Highways: This appendix discusses the findings reported in the research literature for rural highways and limited-access highways regarding the relationships among crashes, operating speed, and roadway characteristics including posted speed limit. ï· Appendix C. Speed Limit Setting Approaches: This appendix presents information on current approaches to setting speed limits including a summary of key publications calling for changes to how speed limits are set. It also provides an overview of various methods being used in the United States and in other countries including introducing recent trends to employ citywide or default speed limits. Existing material and proposed changes to the Manual on Uniform Traffic Control Devices (MUTCD) are summarized. To aid in understanding the relationships between operating speed and posted speed limit, this appendix includes plots showing average speed by posted speed limits. ï· Appendix D. Relationship among Urban/Suburban Roadway Characteristics, Operating Speed, and Crashes in Austin, Texas: This appendix identifies the relationships among operating speed, crashes, and roadway characteristics including posted speed limit and vehicle volume for a database of Austin, Texas, sites. ï· Appendix E. Relationship among Urban/Suburban Roadway Characteristics, Posted Speed Limit, and Crashes in Washtenaw County, Michigan: This appendix identifies the relationships among crashes, roadway characteristics, posted speed limit, and vehicle volume for a database of Washtenaw County, Michigan, sites. ï· Appendix F. Decision Rule Development: This appendix provides an overview of the decision rules selected for the four Speed Limit Setting Groups (SLSGs), which are Limited Access, Undeveloped, Developed, and Full Access. ï· Appendix G. Workshop Slides: This appendix includes the slides and presenter notes used in one of the workshops presented during the project. ï· Abbreviations: This section provides the list of abbreviations used in the report.