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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Posted Speed Limit Setting Procedure and Tool: User Guide. Washington, DC: The National Academies Press. doi: 10.17226/26216.
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1   Several factors are considered within engineering studies when determining the posted speed limit for a speed zone. National Cooperative Highway Research Program (NCHRP) Project 17-76 investigated the factors that influence operating speed and safety and used that knowledge to develop the Speed Limit Setting Procedure (SLS-Procedure) so engi- neers can make informed decisions about the setting of speed limits. The SLS-Procedure was automated with the Speed Limit Setting Tool (SLS-Tool). The SLS-Tool is spreadsheet based and is included with this report for download. Currently, the predominant method for setting speed limits uses the 85th percentile speed. It is viewed as a fair way to set speed limits based on the driving behavior of most drivers (85 percent), who represent reasonable and prudent drivers since the fastest 15 per- cent of drivers are excluded. The 85th percentile speed is also believed to represent a safe speed that would minimize crashes. Criticisms of the 85th percentile speed method include a concern that drivers may not see or be aware of all the conditions present within the cor- ridor, and such an approach may not adequately consider vulnerable roadway users such as pedestrians and bicyclists. Other concerns are that drivers are not always reasonable and prudent, or they only consider what is reasonable and prudent for themselves and not for all users of the system; and the use of measured operating speeds to set speed limits could cause increase speed over time (i.e., speed creep). Drivers frequently select speeds a certain increment above the posted speed limit, anticipating that they will not receive a ticket if they are not above that assumed enforcement speed tolerance. Also, most of the early research justifying the use of the 85th percentile speed was conducted on rural roads; therefore, the 85th percentile speed may not be appropriate for urban roads. The research team considered the breadth of approaches available for the setting of speed limits and the need to develop a methodology that could be used for any roadway type. The research team selected a decision-rule–based procedure for the SLS-Procedure. Given the increased emphasis on designing for the context of the roadway, the research team decided that the SLS-Procedure should be sensitive to context and use the expanded functional classification scheme available in NCHRP Research Report 855 (33). The roadway types and roadway contexts available within the expanded functional classification scheme were collapsed into four Speed Limit Setting Groups (SLSGs): Limited-Access, Undeveloped, Developed, and Full-Access. Unique decision rules were developed for each SLSG. For the SLS-Procedure, the research team proposed consideration of the measured oper- ating speed as the starting point for selecting a posted speed limit but that the measured operating speed be adjusted based on roadway conditions and consideration of the crash experience on the segment. S U M M A R Y Posted Speed Limit Setting Procedure and Tool: User Guide

2 Posted Speed Limit Setting Procedure and Tool: User Guide The guiding principles developed by the research team for the SLS-Procedure included the following: • Use a data-driven approach with research-based decision rules. • Produce consistent results for a given set of conditions. • Incorporate contemporary policies, guidelines, and practices. • Consider drivers’ speed choice and roadway safety. • Provide transparency in the decision-making process. • Consider all roadway types and roadway contexts. • Vary the decision rules to account for the diverse characteristics of each SLSG. • Consider agency data and human resource constraints. • Include inputs and outputs on the same screen to demonstrate the relationship between each roadway characteristic and selection of the suggested speed limit. • Allow for future modifications to accommodate new knowledge. • Create efficiencies in the decision process, where possible. The SLS-Procedure starts with identifying the roadway segment context and type, which determine the appropriate SLSG. For that SLSG, the roadway characteristics and crash potential for the segment are used to identify the speed distribution that should be consid- ered and whether the closest 5-mph increment value or a rounded-down 5-mph increment value should be used. For this project, the research team focused a portion of the Phase II efforts on collecting data for suburban and urban roads to fill the known research gap for city streets. The devel- oped databases for Austin, Texas, and Washtenaw County/Greater Ann Arbor, Michigan, were used to investigate the relationships among crashes, roadway characteristics, and posted speed limits. The team found that crashes on city streets were lowest when the average vehicle operating speed was within 5 mph of the posted speed limit. Therefore, the research team recommended that the 50th percentile speed be a consideration within the SLS-Procedure, especially for the SLSGs of Developed and Full-Access. The evaluation of the Austin, Texas, and Washtenaw County/Greater Ann Arbor, Michigan, data supported including the fol- lowing variables within the decision rules: signal density, access density, and undivided median on four-lane (or more) streets. Findings from the literature were also used to develop the decision rules. Presenting a workshop was a requirement of the research. Members of the research team conducted several workshops and presentations during the development of the SLS-Procedure, and these presentations provided opportunities to obtain feedback on the potential format of the procedure. The presentations with the panel were especially influential in setting the direction for the SLS-Procedure and SLS-Tool. This project concluded with the development of two products: • NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (this document). • Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool is available for download from the TRB website (TRB.org) by searching for “NCHRP Research Report 966.”

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Several factors are considered within engineering studies when determining the posted speed limit, including the 85th percentile speed, which is based on the driving behavior of most drivers (85 percent). The 85th percentile speed is believed to represent a safe speed that would minimize crashes.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide provides and explains a speed limit setting procedure (SLS-Procedure) that considers factors beyond the 85th percentile speed, including both driver speed choice and safety associated with the roadway. This report also provides instructions for using an automated version of the SLS-Procedure via a spreadsheet-based Speed Limit Setting Tool (SLS-Tool). Two versions of the SLS-Tool are available:

N17-76 SLS-Tool (with macros) and

N17-76 SLS-Tool (without macros).

The “without macros” version is made available for users who are not able to use macro codes on their computers. Please see the User Guide for more detailed information on using both versions of the SLS-Tool.

The report is also accompanied by NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool, which documents the research efforts of NCHRP Project 17-76 - Guidance for the Setting of Speed Limits and a Presentation that offers an overview of the project.

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