# 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
Page 10
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.
×

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

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. FigureÂ 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 (TRB.org) by searching for âNCHRP Research Report 966.â 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 FigureÂ 4. Overview of procedure to calculate suggested speed limit.

Next: Section 4 - Decision-Making Steps Within the Suggested Speed Limit Procedure »
Posted Speed Limit Setting Procedure and Tool: User Guide Get This Book
×

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:

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.

1. ×

## Welcome to OpenBook!

You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

Do you want to take a quick tour of the OpenBook's features?

No Thanks Take a Tour »
2. ×

« Back Next »
3. ×

...or use these buttons to go back to the previous chapter or skip to the next one.

« Back Next »
4. ×

Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

« Back Next »
5. ×

To search the entire text of this book, type in your search term here and press Enter.

« Back Next »
6. ×

Share a link to this book page on your preferred social network or via email.

« Back Next »
7. ×

View our suggested citation for this chapter.

« Back Next »
8. ×