National Academies Press: OpenBook

Posted Speed Limit Setting Procedure and Tool: User Guide (2021)

Chapter: Section 3 - Procedure to Calculate the Suggested Speed Limit

« Previous: Section 2 - Speed Limit Relationships and Practices
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Suggested Citation:"Section 3 - Procedure to Calculate the Suggested Speed Limit." 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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10 Overview With consideration of the issues discussed, along with research into the relationships among roadway characteristics including posted speed limit, operating speed, and safety, the research team developed a procedure to calculate a suggested speed limit. The procedure starts with iden- tifying the roadway segment context and type. Next, the speed distribution of drivers on that segment is used to identify a potential suggested speed limit that is adjusted with consideration of the crash potential for the segment. Figure 4 illustrates the steps for the procedure. Additional details are provided in the sections that follow. The suggested speed limit procedure applies to posted speed limits. Procedures for setting school zone, work zone, variable, or advisory speeds are not discussed in this document. Speed Limit Setting Tool The SLS-Tool was developed to facilitate calculating the suggested speed limit. The tool uses spreadsheets to automate the procedure. A copy of the SLS-Tool is available on the TRB website ( by searching for “NCHRP Research Report 966.” S E C T I O N 3 Procedure to Calculate the Suggested Speed Limit Calculated value based on consideration of roadway context and type, speed distribution, and safety Consideration of drivers' speed selection on the segment / Consideration of crash risk based on roadway characteristics Context = rural, rural town, suburban, urban, or urban core / Type = freeway, major arterial, minor arterial, collector, or local Roadway Context and Type Speed Distribution Safety Suggested Speed Limit Figure 4. Overview of procedure to calculate suggested speed limit.

Next: Section 4 - Decision-Making Steps Within the Suggested Speed Limit Procedure »
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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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