nchrp research report 966

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Posted Speed Limit Setting Procedure and Tool: User Guide

This report provides a procedure for setting speed limits and a practitioner-ready user manual explaining the speed limit setting procedure (SLS-Procedure). Additionally, it provides an automated version of the SLS-Procedure via a spreadsheet-based Speed Limit Setting Tool (SLS-Tool). The guidebook will be of interest to engineers responsible for making informed decisions about the setting of speed limits.

• Record URL: http://www.trb.org/Main/Blurbs/182038.aspx
• Find a library where document is available. Order URL: http://worldcat.org/isbn/9780309674041
• Fitzpatrick, Kay
• Das, Subasish
• Pratt, Michael P
• Dixon, Karen
• Publication Date: 2021
• Media Type: Digital/other
• Features: Figures; References; Tables;
• Pagination: 71p
• NCHRP Research Report
• Issue Number: 966
• Publisher: Transportation Research Board
• ISSN: 2572-3766

Subject/Index Terms

• TRT Terms: Automation ; Handbooks ; Methodology ; Speed limits
• Subject Areas: Highways; Operations and Traffic Management; Safety and Human Factors;

Filing Info

• Accession Number: 01774078
• Record Type: Publication
• ISBN: 9780309674041
• Report/Paper Numbers: NCHRP Project 17-76
• Files: TRIS, TRB, ATRI
• Created Date: Jun 12 2021 1:29PM

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Texas A&M Transportation Institute

Saving Lives, Time and Resources.

TTI’s Fitzpatrick Co-Authors NCHRP User Guide on Setting Posted Speed Limits

September 13, 2021

Several engineering factors are considered when determining a posted speed limit, including the 85 th percentile speed, which is based on the driving behavior of most drivers. This percentile speed is believed to represent a speed that would minimize crashes. The report provides and explains a Speed Limit Setting Procedure (SLS-Procedure) that considers factors beyond the 85 th percentile speed, including driver-speed choice and safety associated with the roadway to help engineers make informed decisions about the setting of speed limits. The report also provides instructions for using an automated version of the SLS-Procedure via a spreadsheet-based Speed Limit Setting Tool (SLS-Tool), where users input data for a roadway segment and obtain the suggested speed limit.

“The SLS-Procedure was automated with the SLS-Tool using spreadsheet-based software,” says Fitzpatrick. “The development of the SLS-Tool provides an easy-to-use, intuitive spreadsheet that produces a suggested speed limit reflecting existing conditions. It can also document the conditions for a speed-zone study.”

Interest in the methodology for how speed limits are set is growing in many cities and states that have adopted Vision Zero, an international road traffic safety initiative that aims to achieve no fatalities or serious injuries on roadways. Although several factors influence a driver’s chosen speed, posted speed limits is one of those items. The new SLS-Tool is more sensitive to roadway characteristics and pedestrian and bicyclist activities occurring uniquely within the urban environment.

“The SLS-Procedure and accompanying SLS-Tool reflect current knowledge of relationships among roadway conditions, traffic, safety and operating speeds,” explains Fitzpatrick. “While it represents a step forward with respect to previously available methodologies, the SLS-Tool needs to be kept up-to-date as we learn more about the interconnectivity of those factors.”

The user guide is accompanied by the NCHRP’s 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 . A macro version of the SLS-Tool and a no-macro version are available for download on the NCHRP website .

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Appropriate Speed Limits for All Road Users

There is broad consensus among global roadway safety experts that speed control is one of the most important methods for reducing fatalities and serious injuries. Speed is an especially important factor on non-limited access roadways where vehicles and vulnerable road users mix.

A driver may not see or be aware of the conditions within a corridor, and may drive at a speed that feels reasonable for themselves but may not be for all users of the system, especially vulnerable road users, including children and seniors. A driver traveling at 30 miles per hour who hits a pedestrian has a 45 percent chance of killing or seriously injuring them. 1  At 20 miles per hour, that percentage drops to 5 percent. 1  A number of cities across the United States, including New York, Washington, Seattle and Minneapolis, have reduced their local speed limits in recent years in an effort to reduce fatalities and serious injuries, with most having to secure State legislative authorization to do so.

States and local jurisdictions should set appropriate speed limits to reduce the significant risks drivers impose on others—especially vulnerable road users—and on themselves. Addressing speed is fundamental to the Safe System Approach to making streets safer, and a growing body of research shows that speed limit changes alone can lead to measurable declines in speeds and crashes. 2

Applications

Posted speed limits are often the same as the legislative statutory speed limit. Agencies with designated authorities to set speed limits, which include States, and sometimes local jurisdictions, can establish non-statutory speed limits or designate reduced speed zones, and a growing number are doing so. While non-statutory speed limits must be based on an engineering study, conducted in accordance with the  Manual on Uniform Traffic Control Devices  (MUTCD) involving multiple factors and engineering judgment, FHWA is also encouraging agencies to use the following 3 :

• NCHRP 966: Posted Speed Limit Setting Procedure and Tool
• Safe System approach .

Based on international experience and implementation in the United States, the use of 20 mph speed zones or speed limits in urban core areas where vulnerable users share the road environment with motorists may result in further safety benefits. 4

Considerations

When setting a speed limit, agencies should consider a range of factors such as pedestrian and bicyclist activity, crash history, land use context, intersection spacing, driveway density, roadway geometry, roadside conditions, roadway functional classification, traffic volume, and observed speeds.

To achieve desired speeds, agencies often implement other speed management strategies concurrently with setting speed limits, such as self-enforcing roadways, traffic calming, and speed safety cameras. Additional information is in the following FHWA resources:

• FHWA Speed Management website .
• Self-Enforcing Roadways: A Guidance Report .
• Noteworthy Speed Management Practices .
• Jurisdiction Speed Management Action Plan Development Package .
• Traffic Calming ePrimer .

1. Pilkinton, Paul.  Reducing the speed limit to 20 mph in urban areas: Child deaths and injuries would be decreased . BMJ, Published April 29, 2000.

2. Hu, W. and J. Cicchino (2019).  Lowering the speed limit from 30 to 25 mph in Boston: effects on vehicle speeds . Insurance Institute for Highway Safety.

3. FHWA’s  Methods and Practices for Setting Speed Limits: An Informational Report , (2012).

4.  Recommendations of the Academic Expert Group for the 3rd Global Ministerial Conference on Road Safety .

5. https://highways.dot.gov/safety/speed-management/noteworthy-practice-booklet-speed-management/case-study-7-noteworthy-speed

6. Gayah et al.  Safety and Operational Impacts of Setting Speed Limits below Engineering Recommendations . Accident Analysis and Prevention, Vol. 121, pp.43-52, (2018).

Safety Benefits:

Traffic fatalities in the City of Seattle decreased 26 percent after the city implemented comprehensive, city-wide speed management strategies and countermeasures inspired by Vision Zero. This included setting speed limits on all non-arterial streets at 20 mph and 200 miles of arterial streets at 25 mph. 5

One study found that on rural roads, when considering other relevant factors in the engineering study along with the speed distribution, setting a speed limit no more than 5 mph below the 85th-percentile speed may result in fewer total and fatal plus injury crashes, and lead to drivers complying closely with the posted speed limit. 6

The objectives of this research were to (1) identify and describe factors that influence operating speed and (2) provide guidelines to make informed decisions related to establishing speed limits on roadways. The guidelines are to address the following: (a) an approach for determining the relationship between operating, design, posted and statutory speeds and how the differences among them impact safety performance; (b) identification and classification of nationwide data including, but not limited to, geometric design, access density, signal density, traffic volume characteristics, and enforcement practices that may impact operating speed; (c) an analysis of the 85 th percentile speed and other statistical measures and factors as appropriate methods for setting speed limits; (d) implications of setting a speed limit lower than those recommended using the factors identified above. The focus of the research was on roadway functional classifications of minor arterials and higher as defined by AASHTO. STATUS:  Research is complete. The final report is available as NCHRP Report 966 at https://www.nap.edu/catalog/26216/posted-speed-limit-setting-procedure-and-tool-user-guide , along with associated deliverables.

Development of a Posted Speed Limit Setting Procedure and Tool (2021)

Chapter: summary.

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.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 1 SUMMARY 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 as well as safety and used that knowledge to develop the Speed Limit Setting Procedure (SLS-Procedure) so engineers 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 the NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (1). Currently, the predominant method for setting speed limits uses the 85th percentile speed. It is viewed as being 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 percent 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 corridor, 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 increased 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 841: Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments. The roadway types and roadway contexts available within the expended 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 recommended consideration of the measured operating 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. The guiding principles developed by the research team for the SLS-Procedure included that the procedure:  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 process.

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 2  Consider all roadway types and roadway contexts.  Vary the decision rules to account for the diverse characteristics of each speed limit setting group.  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 considered and whether the closest 5-mph increment value or a rounded-down 5-mph increment value should be used. Based on the previous steps, a potential suggested speed limit is identified. Figure 1 illustrates the procedure. Figure 1. Overview of SLS-Procedure to calculate the suggested speed limit. In NCHRP Project 17-76, 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 developed databases for Austin, Texas, and Washtenaw, 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 operating speed was within 5 mph of the average operating speed. 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, Michigan, data supported including the following 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. Calculated value based on  consideration of roadway  context & 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

NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 3 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. NCHRP Project 17-76 concluded with the development of two publications:  NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide.  NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool (this document).

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.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool documents the research efforts and findings from an NCHRP Project 17-76 to identify factors that influence a driver’s operating speed and the development of a Speed Limit Setting Procedure and Tool.

The document is supplemental to NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide .

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