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
                        Applying Safety Data and Analysis to Performance – Based Transportation Planning


            
            In This Section
        Zero Deaths and Safe System
                Safety Culture
                FHWA's Commitment to Safety
                National Efforts
                Resources
                Equity in Roadway Safety
                                Equity in Roadway Safety Overview and Resources
                Collect and Analyze Safety Data Equitably
                Engage Community Representatives
                Implement Safety Improvements Equitably
                Evaluate Safety Improvements Equitably
                Vision Zero Community of Practice
                                About
                News Flash and Events
                Planning
                Funding
                Implementation
                Resources
                Vision Zero Action Plans
                News From Vision Zero Communities
                Community Pairing Program
                
                Appendix D. Regression to the Mean
From year to year, the number of crashes at a site will randomly fluctuate up and down. Overtime, however, this random fluctuation will balance out to what can be considered the long-term expected average number of crashes at the site. Figure D.1 demonstrates regression to the mean and the effects of average crash frequency across multiple years. The ‘Crashes’ line shows hypothetical 1990 to 2010 annual crash frequency at a site. The crash frequency varies up and down from year to year. The ‘Mean’ line represents the long-term average crash frequency at the same hypothetical site. As shown, the five-year rolling average stabilizes at approximately 14 crashes per year. For example, the first five-year average is from 1990 to 1994 and is plotted in 1994, the second is from 1991 to 1995 and plotted on 1995. The five-year rolling average more closely approximates the long-term average then the annual crash frequency alone.

Figure D.1 Chart. Example of Regression to the Mean



Source: Federal Highway Administration. Improving Safety on Rural Local and Tribal Roads: Safety Toolkit. August 2014.

If regression to the mean is not accounted for, a site might be selected for study because the annual number of crashes that occurred was higher than “usual” due to a random fluctuation in the data. Conversely, a site that should be selected for study might be overlooked because an unusually low number of annual crashes occurred there.

To reduce the influence of regression to the mean, the agency should calculate the average of the most recent three to five years of crash data to determine the average crash frequencies. This minimizes year-to-year fluctuations in data and is appropriate if site conditions (e.g., traffic volume, land use, driveway access, and roadway configuration) have not changed. However, if site conditions have changed significantly during the analysis period, it may be more appropriate to monitor the site and evaluate safety after conditions have stabilized.
      
  Book traversal links for Appendix D. Regression to the Mean
      ‹ Appendix C. Risk Ratio
                    Up
                
                Book:Applying Safety Data and Analysis to Performance – Based Transportation Planning
              1.0 Introduction
                2.0 Audience, Purpose, and Guidebook Structure
                3.0 Performance-Based Planning Processes
                4.0 Getting Started with Transportation Safety Planning—Data Collection
                5.0 Using Safety Analyses for Planning
                7.0 Conclusions
                Appendix B. Safety Analysis Objectives and Methods
                Appendix C. Risk Ratio
                Appendix D. Regression to the Mean 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