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 C. Risk Ratio
The risk ratio compares the severity of crashes associated with a particular factor to the severity of all other crashes (e.g., the percentage of angle crashes that result in a serious injury or fatality divided by the percentage among all other crashes, excluding angle crashes). Crash types or factors with a risk ratio greater than 1 are overrepresented with respect to severe crashes. Formally, the risk ratio is defined by the following equation.

Figure C.1 Equation. Risk Ratio



Where:

A = the number of severe crashes of a particular type or emphasis area;

B = the total number of nonsevere crashes of the same type as in A;

C = the number of severe crashes, excluding those of the same type as in A; and

D = the total number of nonsevere crashes, excluding those of the same type as in A.

To illustrate this concept further, a few examples are provided below. Conventionally, data used to compute the risk ratio (variables A, B, C, and D) are arranged in a 2×2 matrix, where the first row corresponds to the numerator values in the formula above, and the second row corresponds to the denominator values.

Table C.1 Risk Ratio Data for Crashes in Rural Areas
	Empty cell
			Severe Crash
			Not a Severe Crash
			Total CrashesRural Area
			523 (A)
			5,768 (B)
			6,291 (A+B)Not a Rural Area (Urban Area)
			700 (C)
			24,396 (D)
			25,096 (C+D)Source: Sample data, Cambridge Systematics, Inc., 2015.

Given this data, the formula to calculate the risk ratio is as follows:

Figure C.2 Equation. Severe Crash Risk Ratio for Rural Areas



Based on this calculation, crashes in rural areas are around three times more likely to result in a fatality or serious injury than those in urban areas.

A similar example for DUI crashes is provided here:

Table C.2 Risk Ratio Data for DUI Crashes
	Empty cell
			Severe Crash
			Not a Severe Crash
			Total CrashesDUI
			201 (A)
			1,589 (B)
			1,790 (A+B)Not a DUI
			1,022 (C)
			28,575 (D)
			29,597 (C+D)Source: Sample data, Cambridge Systematics, Inc., 2015.

Figure C.3 Equation. Severe Crash Risk Ratio for DUI Crashes



A risk ratio can also be developed for a combination of factors. An example for DUI crashes in rural areas is shown here:

Table C.3 Risk Ratio Data for Rural DUI Crashes
	Empty cell
			Severe Crash
			Not a Severe Crash
			Total CrashesRural DUI Crash
			93 (A)
			597 (B)
			690 (A+B)Not a Rural DUI Crash
			1,130 (C)
			29,567 (D)
			30,697 (C+D)Source: Sample data, Cambridge Systematics, Inc., 2015.

Figure C.4 Equation. Severe Crash Risk Ratio for DUI Crashes in Rural Areas



As an outcome of this step, the transportation planner would understand the categories of crashes by type, severity, contributing factor, or geography that may be a focus for planning and programming or that should be a consideration in nonsafety-specific projects.
      
  Book traversal links for Appendix C. Risk Ratio
      ‹ Appendix B. Safety Analysis Objectives and Methods
                    Up
                    Appendix D. Regression to the Mean ›
                
                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