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

Chapter: Acronyms and Abbreviations

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Page 55
Suggested Citation:"Acronyms and Abbreviations." 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.
Page 55

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55   AADT Average annual daily traffic AASHTO American Association of State Highway and Transportation Officials ADT Average daily traffic C50 The 5-mph increment that is closest to the 50th percentile speed C85 The 5-mph increment that is closest to the 85th percentile speed CMF Crash modification factor Expanded FCS Expanded Functional Classification System FHWA Federal Highway Administration HSIS Highway Safety Information System HSM Highway Safety Manual ISW Inside shoulder width K Constant associated with the confidence level (1.645 for 95 percent confidence) KABC Fatal and injury crash severity levels KABCO All crash severity levels LW Lane width M Exposure (100 million vehicle miles) mph Miles per hour MUTCD Manual on Uniform Traffic Control Devices MVM Million vehicle miles N Number of lanes NCHRP National Cooperative Highway Research Program NTSB National Transportation Safety Board Ra Average crash rate for a given road type, provided by the user or obtained from HSIS tables Rc Critical crash rate for a given road type RD50 The 5-mph increment obtained by rounding down the 50th percentile to the nearest 5-mph increment RD85 The 5-mph increment obtained by rounding down the 85th percentile to the nearest 5-mph increment SLS-Procedure Speed Limit Setting Procedure SLS-Tool Speed Limit Setting Tool SLSG Speed Limit Setting Group SW Shoulder width TCD Traffic control device TWLTL Two-way left-turn lane Acronyms and Abbreviations

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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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