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

Chapter: Section 1 - Introduction

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Suggested Citation:"Section 1 - Introduction." 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:"Section 1 - Introduction." 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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3   Background The speed limit is the maximum speed legally permitted for a given roadway segment. Several types of speed limits exist, including statutory speed limit, posted speed limit, school zone speed limit, work zone speed limit, variable speed limit, and advisory speed limit. (Figure 1 illustrates these different types 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 roads—such as freeways, rural highways, or urban streets—which are applicable even if the speed limit sign is not posted. Objective The National Cooperative Highway Research Program (NCHRP) Project 17-76 research team was tasked with identifying factors that influence a driver’s operating speed and then developing a Speed Limit Setting Procedure (SLS-Procedure) and automating the SLS- Procedure with a 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 objective suggested speed limit value. Traffic engineers can use the SLS-Procedure and the suggested speed limit generated by the SLS-Tool to com- municate with the public or government officials to explain the general procedures behind setting speed limits. The products developed through NCHRP Project 17-76 focused on posted speed limits and not on other types of speed limits (see Figure 1 for examples). The SLS-Tool is designed to cover the most frequently encountered road designs and settings, though there may be circumstances not covered by the SLS-Tool that will require additional engineering judg- ment in the selection of the appropriate posted speed limit. Two products were generated as part of this project: • NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (this document). • NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool (2). S E C T I O N 1 Introduction

4 Posted Speed Limit Setting Procedure and Tool: User Guide Organization of User Guide This document is the user guide for the SLS-Procedure and SLS-Tool. It contains the fol- lowing sections: • Section 1: Introduction: provides an overview of the document including the project objec- tives and the organization of the guide. • Section 2: Speed Limit Relationships and Practices: introduces several basic relationships with regard to speed limits. • Section 3: Procedure to Calculate the Suggested Speed Limit: presents the procedure to develop a suggested speed limit for a corridor. • Section 4: Decision-Making Steps Within the Suggested Speed Limit Procedure: docu- ments the four decision-making steps, which include selecting roadway segment context and type, identifying the appropriate speed distribution, adjusting for safety considerations, and finally calculating the suggested speed limit. • Section 5: Variables for Decision-Making Procedure: discusses each variable used within the decision-making procedure (i.e., the SLS-Procedure). • Section 6: Speed Limit Setting Tool: provides an overview of the SLS-Tool, including data entry requirements, messages that may be generated, and default values if data are not avail- able for one of the variables. • Section 7: SLS-Tool Case Study Examples: presents a case study for each of the four Speed Limit Setting Groups (SLSGs). • Section 8: Other Considerations When Setting Posted Speed Limits: discusses several issues associated with the setting of posted speed limits. • Section 9: Related Reference Materials: lists other reference materials on posted speed limits including links when available. • Acronyms and Abbreviations: lists the acronyms and abbreviations used within this user guide. • References: provides details on the material referenced in this user guide. Source: Federal Highway Administration, Speed Limit Basics, page 1 (1). Figure 1. Examples of speed limits.

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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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