USA Banner
  U.S. flag
              An official website of the United States government Here's how you knowOfficial US Government Icon
              **Official websites use .gov**A **.gov** website belongs to an official government organization in the United States.
            Secure Site Icon
              **Secure .gov websites use HTTPS**
                  A **lock** ( A locked padlock ) or **https://** means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites. 
 U.S. Department of Transportation Icon United States Department of Transportation
 Search DOT
            Search
                    About FHWAFHWA Organization
                Headquarters Offices
                Field Offices
                Strategic Plan
                Business Opportunities
                Staff Directories
                Careers
                Highway History
                Contact UsProgramsAcquisition Management
                Civil Rights
                Federal Lands Highway
                Infrastructure
                Innovative Program Delivery
                Operations
                Planning, Environment, and Realty
                Policy
                Research, Development, and Technology
                Safety
                Browse by TopicsResourcesCore Highway Topics
                Federal Aid Essentials
                Automation
                Complete Streets
                Laws and Regulations
                Policy & Guidance Center
                Section 508
                Publications and Statistics
                Professional Development
                National Highway Institute
                Resource Center
                Accessibility Resource Library
                FHWA Research LibraryNewsroomPress Releases
                Speeches & Testimony
                Photos
                Videos
                Media ContactsMenu FHWA Highway Safety Programs Home
                        Methods and Practices for Setting Speed Limits: An Informational Report


            
            In This Section
        Speed Management Safety
                USLIMITS2
                                Overview
                Create a New Project
                Revise an Existing Project
                Frequently Asked Questions
                Technical Support
                Facts & Statistics
                Engineering Speed Limits
                Traffic Calming ePrimer
                Ongoing Research
                Reference Materials
                Related Web Site Links
                
                Appendix A: Glossary
The following definitions are provided to aid in the understanding of setting speed limits. They may or may not coincide with terms and definitions found in related State statutes.

**10 mph Pace:**The 10 mph pace is the 10 mph range encompassing the greatest percentage of all the measured speeds in a spot speed study.

**85th Percentile Speed:**The 85th percentile speed is the speed at or below which 85 percent of the free-flowing vehicles travel.

**Advisory Speed:**Advisory speeds warn drivers to proceed at a speed lower than the speed limit due to geometrics, surface, sight distance, or other conditions.

**Annual Average Daily Traffic:**Commonly abbreviated as AADT, the total number of vehicles traversing a point or facility in one year divided by 365.

**Average Speed:**The average (or mean) speed is the most common measure of central tendency. Using data from a spot speed study, the average is calculated by summing all the measured speeds and dividing by the sample size.

**Design Speed:**The design speed is a selected speed used to determine the various geometric design features of the roadway.

**Differential Speed Limit:**A system that prescribes different maximum speed limits for different vehicle types or user groups. This is usually applied as one maximum speed limit for light passenger vehicles, and a lower maximum speed limit for trucks and heavy commercial vehicles.

**Free-flow Speed:**Free-flow speed is the speed a driver chooses when there are no influences from other vehicles, conspicuous enforcement, or environmental factors; in other words, this is the speed the driver finds comfortable based on the appearance of the road.

**Injury Minimization Speed Limit:**Also known as a speed limit for safe systems, it is a speed limit that is set so that the forces experienced by the human body in the event of a crash will not exceed biomechanical tolerances resulting in death or a severe personal injury.

**Optimal Speed Limit:**A speed limit that yields the minimum total cost to society, including vehicle operating costs, crash costs, travel time costs, and other societal costs.

**Rational Speed Limit:**A speed limit that is based on a formal, analytical review of traffic flow, roadway design, local development, and crash data. For existing roads, it uses the 85th percentile speed of free-flowing vehicles operating under normal traffic, weather, and roadway conditions as the speed limit, adjusted down by factors that can affect safety, such as road design features and roadside development and are not readily apparent to the motorist. The analysis also considers crash history and the influence of speed as a contributing factor. The 85th percentile speed is based on the premise that the vast majority of drivers will select a speed that is reasonable, safe, and prudent for a given road. Drivers who exceed the 90th percentile have a significantly higher risk of crashing.

**Road Safety Audit:**A formal safety performance examination of an existing or future road or intersection by an independent audit team.

**Speed Dispersion:**The speed dispersion refers to the normal spread in vehicle speeds observed in a study section.

**Speed Limit, Absolute:**An absolute speed limit is a numerical value, the exceeding of which is always in violation of the law, regardless of the conditions or hazards involved.

**Speed Limits, Environment:**An environmental speed limit is a speed limit created for the purpose of meeting federal air quality standards.19

**Speed Limit, Posted:**The posted speed limit is the value conveyed to the motorist on a black-on-white regulatory sign. Standard engineering practice is to post speed limits for freeways, arterials, and any roadway or street where speed zoning has altered the limit from the statutory value.

**Speed Limit, Prima Facie:**A prima facie speed limit is one above which drivers are presumed to be driving unlawfully. Nevertheless, if charged with a violation, drivers have the opportunity to demonstrate in court that their speed was safe for conditions at the time and not in violation of the speed limit, even though they may have exceeded the numerical limit.

**Speed Limits, Statutory:**Numerical speed limits specifically provided for under a State's traffic codes that apply to various classes or categories of roads (e.g., rural expressways, residential streets, primary arterials, etc.). State laws may or may not require that these limits be posted.15

**Speed Zoning:**Speed zoning is the process of performing and engineering a study and establishing a reasonable and safe speed limit for a section of roadway where the statutory speed limits given in the motor vehicle laws do not fit the road or traffic conditions at a specific location.

**Speeding:**The legal definition of speeding is exceeding the posted speed limit. In the road safety community, speeding is defined as exceeding the posted speed limit or speed too fast for conditions.

**Test Run:**A speed test run is performed by driving through a study area (potential speed zone) at a reasonable free-flow speed and collecting speed data, then using this data to confirm speed limits or speed data collected from other vehicles in the study area.
      
  Book traversal links for Appendix A: Glossary
      ‹ References
                    Up
                    Appendix B: Example Traffic Control Order ›
                
                Book:Methods and Practices for Setting Speed Limits: An Informational Report
              Background
                Purpose & Scope
                Report Organization
                The Safety of Speed
                Speed Limit Basics
                Speed Limit Sign and Placement
                Speed Study Data Collection
                Speed Limit Enforcement
                Case Studies
                Summary of Results
                References
                Appendix A: Glossary
                Appendix B: Example Traffic Control Order
                Appendix C: Illinois Policy on Setting Speed Limits
                Appendix D: Northwestern Speed Zoning Technique
                Appendix E: Speed Limits New Zealand (Road Risk Methodology) U.S. DEPARTMENT OF TRANSPORTATION

**Federal Highway Administration**

1200 NEW JERSEY AVENUE, SE

WASHINGTON, DC 20590

202-366-4000Subscribe to email updates
                    About
  

        
              About FHWA
                Careers
                Org Chart
                Staff Directories
                Work with UsPrograms
  

        
              Acquisition Management
                Civil Rights
                Federal Lands Highway
                Infrastructure
                Innovative Program Delivery
                Operations
                Planning, Environment, and Realty
                Policy
                Research, Development, and Technology
                SafetyPolicies, Rights, Legal
  

        
              About DOT
                Accessibility
                Budget and Performance
                Civil Rights
                FOIA
                Information Quality
                No FEAR Act
                Office of Inspector General
                Privacy Policy
                USA.gov
                Web Policies and Notices
                Web StandardsNews & Events
  

        
              Newsroom
                Press Releases
                Speeches & Testimony
                Media Contacts
                Connect with Us