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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Nchrp fy 2025 program announcement and solicitation for panel nominations.

21 May 2024

Panel Nominations Due by June 30, 2024

On April 11-12, 2024, the AASHTO Special Committee on Research and Innovation (R&I) formulated NCHRP's FY 2025 Program . Although the FY 2025 projects are tentative (they have not yet been approved by States' ballot), we are soliciting now for nominees to meet future deadlines.

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Guidance for the Setting of Speed Limits

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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.

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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

• View Record

Research in Progress

Guide for Self-Explaining Roads in the Context of the Safe System Approach

Self-explaining roads (SERs), also known as self-enforcing roads, use the design of the roadway itself to achieve the goals of a proactive and equitable Safe System Approach. SERs, designed to be recognizable, distinguishable, interpretable, and safe, can be effective in terms of managing speed, thereby reducing the occurrence of fatal and serious injury crashes. Two fundamental criteria a SER must meet are homogeneity (sameness) within and heterogeneity (differences) between roadway classifications and contexts to increase predictability within the road environment. Roadway characteristics and competing stakeholder demands often undermine the application of design and operational characteristics and features, partially because the impacts of those features are not well understood, communicated, or implemented in the context of improving safety for all road users. A SER reinforces the desired functions of a facility by emphasizing visual differences between roadway categories that are often categorized by multimodal considerations and speed [e.g., 0-25 miles per hour (mph), 25-40 mph, 40+ mph]. Defining safe speeds and setting credible speed limits have a long history, with many studies exploring the impact of specific features on driver speed profiles, including the most recent ones such as NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (2021) and FHWA-HRT-17-098 Self-Enforcing Roadway, A Guidance Report (2018). The vast but unconsolidated literature on speed lacks a format easily applicable by professionals in safety, planning, design, and operations. This research is necessary to bridge the gap between the SER concept and its implementation in different functional classes and contexts, and identify the critical SER features so that practitioners can be more intentional and cognizant of the impacts of their decisions on all road users. Specifically, the final product of the project is expected to include recommendations that support active transportation and can be applied during the various phases of project development. These phases include geometric design, active transportation facilities, roadside design, land use, and traffic control devices, resulting in operating speeds that closely match target speeds. The objective of this research is to develop a guide for applying the SER concept to transportation planning, design, and operations to improve multimodal user safety, especially for vulnerable road users (VRUs) on nonfreeway arterial and collector systems in urban and suburban areas.

• Record URL: http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5493
• Contract to a Performing Organization has not yet been awarded.
• Status: Proposed
• Funding: $700000

Project 07-36

National Cooperative Highway Research Program

American Association of State Highway and Transportation Officials (AASHTO)

Federal Highway Administration

• Start Date: 20240507
• Expected Completion Date: 0
• Actual Completion Date: 0

Subject/Index Terms

• TRT Terms: Alternatives analysis ; Crash rates ; Highway design ; Highway operations ; Highway safety ; Injury severity ; Speed limits ; Traffic calming ; Traffic speed
• Subject Areas: Design; Highways; Operations and Traffic Management; Safety and Human Factors;

Filing Info

• Accession Number: 01888644
• Record Type: Research project
• Source Agency: Transportation Research Board
• Contract Numbers: Project 07-36
• Files: TRB, RIP
• Created Date: Jul 25 2023 8:50AM

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

Chapter: section 6 - speed limit setting tool.

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.

33   Overview of SLS-Tool Requirements The SLS-Tool is designed to help practitioners assess and establish consistent speed limits for segments of streets and highways. The tool combines customary engineering studies with context-sensitive considerations to identify appropriate speed limits. The engineering studies typically include evaluating criteria such as 85th percentile speed, traffic volume, number of access points, bicyclist activity, pedestrian activity, crash history, and others. The SLS-Tool is designed to produce an unbiased and objective suggested speed limit value based on the 50th and 85th percentile speed, roadway characteristics, and safety. The SLS-Tool is an Excel®-based spreadsheet program that provides an objective suggested speed limit that traffic engineers can use to communicate with the public or government offi- cials to explain the methodology behind setting speed limits. The tool provides the rationale for setting the speed limit based on key site characteristics, including the statutory speed limit, the distribution of traffic speed, site characteristics, and crash data. Two versions of the SLS-Tool are available: • N17-76 SLS-Tool (macro). • N17-76 SLS-Tool (no macro). The N17-76 SLS-Tool (macro) uses macro code to display the required data input cells for the specified roadway context and type. This worksheet contains a single analysis worksheet that is used for all SLSGs. The macro code displays only the required data entry rows for the roadway context and type specified by the user. The macro code also includes control buttons that allow the user to clear the data from the Analysis worksheet or populate the data entry cells with a set of default values. When the user opens the tool, Excel® may display a security message indicat- ing that macro code has been disabled. The user must click the “Enable Content” button that appears in a yellow ribbon on the top of the screen. It may also be necessary to check the macro security settings as follows: 1. Select “File” in the upper ribbon. 2. Select “Options.” 3. Select “Trust Center.” 4. Click the “Trust Center Settings” button. 5. Select “Macro Settings.” 6. If the option of “Disable all macros without notification” is selected, select a different option and click the “OK” button. The other version of the SLS-Tool—N17-76 SLS-Tool (no macro)—does not use macro code. This version is available for users who are not able to use macro codes on their computers. The no-macro version contains one analysis worksheet for each SLSG (Limited-Access, Developed, S E C T I O N 6 Speed Limit Setting Tool

34 Posted Speed Limit Setting Procedure and Tool: User Guide Undeveloped, and Full-Access), and the user must select the appropriate worksheet for each analyzed segment. This version does not provide control buttons to clear data or populate the data entry cells with default values. Data Entry The main data entry area is located in columns A–F of the worksheet. The data entry area is organized with boxes for the following data categories: • Site Description Data: Enter basic roadway characteristics such as the roadway context and type, indicate whether crash data are available for the analysis, and enter optional information such as the user’s name, analysis date, and roadway segment location. • Analysis Results: The SLS-Tool provides the roadway group (Limited-Access, Developed, Undeveloped, or Full-Access) based on the specified roadway context and type, and displays the suggested speed limit. • Speed Data: Enter the maximum (statutory) speed limit and the needed speed distribution values. The 50th percentile speed is needed for all roadway groups, and the 85th percentile speed is needed for all roadway groups except Full-Access. • Site Characteristics: Enter data to specify the segment length, AADT (two-way total), number of lanes, and other attributes describing the segment’s design and traffic control characteristics. • Crash Data: If crash data are available, enter data to specify the time period, traffic volume, and crash counts. Most of the cells in the SLS-Tool are locked to prevent the user from altering equations and obtaining inaccurate results. Data entry cells are unlocked, and many of the cells have drop- down menus that contain the valid entry options for the cell. For example, the roadway context cell is limited to the categories listed in Figure 7 (Rural, Rural Town, Suburban, Urban, and Urban Core). The data entry cells are color coded to help the user understand the type of data needed. The following colors are used: • Aqua: basic input cell. • Denim: basic input cell with drop-down menu. • Orange: optional input cell (not needed for calculations). These cells include the user’s name, analysis date, roadway name and description, current speed limit, and notes. The user may enter this information for documentation purposes if desired. • Green: optional input cell. These cells contain values that are used for calculations but should be left blank if values are not available. Specifically, the user may enter average crash rates for segments like the one being analyzed, but the SLS-Tool can also estimate average crash rates if the user lacks data to provide average crash rates. • Rose: intermediate calculations. • Purple: final analysis results (specifically, the suggested speed limit). • Yellow: calibration coefficient or policy value. The user should change these cells only based on actual data (e.g., crash rates for specified roadway types) or documented policies (e.g., statutory minimum and maximum speed limits). Select values used in the analysis calculations are in the “Support Tables” worksheet. That worksheet includes the assumed values for minimum segment lengths by speed limits, upper and lower speed limits by roadway group, SLSGs by roadway type and roadway context, and HSIS crash rates. Intermediate calculation cells are located to the right of the data entry area. Users will not need to use these cells.

Speed Limit Setting Tool 35   A legend is provided on the top portion of the main data entry area to summarize the color- coding patterns. A button labeled “Clear all data” is also provided to allow the user to clear input data and restart the analysis with a blank worksheet. When this button is clicked, a message box appears and asks, “Clear all input data?” Click yes to clear the data or no to cancel the operation. After clicking yes, a second message box appears and asks, “Enter default values into data entry cells?” Click yes to populate the data entry cells with default values or no to leave the cells blank. Advisory, Calculated, or Warning Messages The SLS-Tool checks for several conditions and issues messages as needed. Table 20 lists the conditions along with the advisory, calculated, or warning messages. These messages are color coded to indicate the message types as follows: • Advisory message: blue font, used to call attention to issues that are not errors but could be improved. • Calculated message: purple font, used to describe calculation results. • Warning message: red font, used to call attention to erroneous input data. Condition Message Missing required data Enter values for all variables marked with O. (An O will appear to the right of empty input cells.) Missing roadway context or roadway type Specify roadway context and roadway type in cells B5 and B6. Completed calculations This value is determined by <x>. (The quantity x is specified as the maximum speed limit, speed data, site characteristics, and/or crash data, depending on which variables governed the setting of the speed limit.) Completed calculations but with maximum speed limit out of range (too high) The calculated value exceeds the upper value for this speed limit setting group; therefore, the suggested speed limit reflects the assumed upper value. Completed calculations but with maximum speed limit out of range (too low) The calculated value is below the lower value for this speed limit setting group; therefore, the suggested speed limit reflects the assumed lower value. Maximum speed limit out of range (too high) The assumed upper value for this speed limit setting group is <max> mph. Maximum speed limit out of range (too low) The assumed lower value for this speed limit setting group is <min> mph. 50th percentile speed is greater than 85th percentile speed The 85th percentile must be greater than the 50th percentile. 85th percentile speed is only 1 mph greater than 50th percentile speed (suggesting a very tight speed distribution) The 85th percentile is only 1 mph greater than 50th percentile. Interpret results with caution. Segment length < Minimum_Segment_Length For a suggested speed limit of x mph, minimum segment length = y mi. Adverse alignment present Consider location-specific advisory speed warnings. Less than 1 year of crash data Calculations based on 1 year of crash data or less and should be interpreted with caution. Less than 3 years of crash data Consider collecting at least 3 years of crash data. Average crash rates are greater than computed critical crash rates Critical rates should be higher than average rates. The entered number of KABC crashes is greater than the entered number of KABCO crashes The number of KABC crashes must be less than or equal to the number of KABCO crashes. Crash rates are calculated from input data Observed/average KABCO/KABC crash rate = x crashes/100 MVM. (For average crash rates, the message will also specify “from User” if the user provided the rate or “from HSIS” if the user did not provide the rate.) Input data value justifies lowering the speed limit below the closest 85th percentile value Rounded-down 85th, closest 50th, or rounded-down 50th percentile value. Table 20. SLS-Tool advisory, calculated, or warning messages.

36 Posted Speed Limit Setting Procedure and Tool: User Guide SLSG Upper Speed Limit Checka Lower Speed Limit Check Limited-Access Depends on the state. The SLS-Tool has 85 mph as the upper limit because it is the highest currently allowed in the United States. 50 Undeveloped Depends on the state. The SLS-Tool has 70mph. 25 Developed 55 25 Full-Access 30 15 aUse the maximum speed provided by the user if the user-provided speed is lower than the value in this table. Table 21. Upper and lower speed limit checks by Speed Limit Setting Group. Several messages refer to the upper and lower speed limit values for the relevant roadway group. Table 21 provides these upper and lower values. The values can be altered in the yellow table in the “Calibration Tables” worksheet if needed. For example, if the segment of interest is an undeveloped facility in a jurisdiction that has a maximum speed limit of 75 mph for these types of facilities, then the user can enter 75 mph into the appropriate cell in the yellow table. How to Handle Situations When Data Are Not Available for One of the Variables Some of the variables are fundamental quantities that must be provided for all analysis cases. These variables include roadway context and roadway type. For all other variables, default values can be used if actual data are not available. Table 22 shows these values for speed- and geometric- related variables, and Table 23 shows values for crash-related variables. Variable Roadway Group Default Value 50th percentile speed All Maximum speed limit – 5 mph 85th percentile speed Limited-Access, Developed, or Undeveloped Maximum speed limit – 2 mph AADT (two-way total) Developed 30,000 veh/d AADT (two-way total) Full-Access 10,000 veh/d AADT (two-way total) Limited-Access (roadway context = rural) 25,000 veh/d AADT (two-way total) Limited-Access (roadway context = urban) 60,000 veh/d AADT (two-way total) Undeveloped 15,000 veh/d Angle parking present Developed or Full-Access No Bicyclist activity Developed or Full-Access Not high Design speed Limited-Access ≥ 60 mph Directional design-hour truck volume Limited-Access 200 trucks/hr Grade Limited-Access 0% ISW Limited-Access 6 ft Lane width Undeveloped 12 ft Maximum speed limit All See Table 21 Median type Developed or Full-Access Divided Median type Undeveloped Divided Number of access points Developed 40 access points Number of access points Full-Access 60 access points Number of access points Undeveloped 15 access points Number of interchanges Limited-Access 7 (1 interchange/mi × 7 mi) Number of lanes Developed, Undeveloped, or Full- Access 4 lanes Number of lanes Limited-Access 6 lanes Table 22. Input data default values for speed and geometric-related variables.

Speed Limit Setting Tool 37   The default values are chosen to reflect ideal conditions. That is, a site with conditions equal to the default values will have its speed limit set based on the closest 85th percentile speed. The user must enter any data values that deviate from ideal conditions, which may result in setting the speed limit based on a lower speed. Table 23. Input data default values for crash-related variables. Variable Roadway Group Default Value Crash data availability All Yes Number of years of crash data All 3 years Is the segment a one-way street? Developed or Full-Access No Average AADT (two-way total) for crash data period All Same as AADT for site characteristics data All (KABCO) crashes for crash data period All Number needed to yield a crash rate equal to 1/3 that of the HSIS-based average rate Fatal and injury (KABC) crashes for crash data period All Number needed to yield a crash rate equal to 1/3 that of the HSIS-based average rate Table 22. (Continued) Variable Roadway Group Default Value Number of traffic signals Developed 3 signals Number of traffic signals Full-Access 8 signals On-street parking activity Developed or Full-Access Not high Outside SW Limited-Access 10 ft Parallel parking permitted? Developed No Pedestrian activity Developed or Full-Access Negligible Segment length Developed or Full-Access 1 mi Segment length Limited-Access or Undeveloped 7 mi SW Undeveloped 10 ft Sidewalk buffer Developed or Full-Access Present Sidewalk presence/width Developed or Full-Access Adequate

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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