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
                        Safety
                        Other Safety Areas


            
            In This Section
        Other Safety Areas
                Road Diets
                Lighting and Visibility
                                Overview
                Sign Retroreflectivity
                Pavement Markings
                Roadway Lighting
                Focused Approach to Safety
                Motorcyclist Safety
                Older Road Users
                                Overview
                Crash Facts
                Engineering and Training
                Federal and State Programs
                Community Resources
                Articles
                Work Zone Safety
                Transportation Safety Planning (TSP)
                
                Crash Modification Factors (CMFs)
cmf.pdf (410.87 KB)
    
    
          FHWA-SA-14-083

Introduction

Local and rural road owners often have limited financial resources available to implement highway safety improvements. Therefore, it is important that safety improvements return the highest level of benefit for each dollar invested. A primary benefit of safety improvements is to reduce crashes and fatalities, so it is useful for local and rural road owners to understand how much a particular safety improvement, or set of safety improvements, can reduce crashes. Published resources are available to assist local and rural road owners in understanding the crash reduction potential associated with specific safety improvements. This briefing sheet describes these resources and provides an example of how a crash modification factor can be used to assess the safety impact of a set of improvements.

Developing CMFs

Highway safety professionals have conducted numerous studies measuring the crash reduction potential of various types of safety improvements. Many of these estimates have been developed by comparing crashes "before" implementation of a safety improvement against crashes "after" implementation. The measured change in crashes is used to develop a "crash modification factor," or CMF. A CMF is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site.

While some experience and judgment is required to develop and apply CMFs, the information derived from their proper application can benefit local and rural road owners in selecting safety improvements or "countermeasures" by providing a basis to understand how crashes are affected by a particular safety improvement or set of improvements.

Resources

Application of CMFs requires an appreciation of their sources and limitations. The CMF Clearinghouse1 contains over 3,000 CMFs, each developed through one or more safety studies. The Clearinghouse provides a searchable database for CMFs and accompanying background information on each. The practitioner can use the search tools available with the Clearinghouse to find the CMFs that match the facility where they have a safety need; e.g., intersections or roadway segments. Each CMF has a "star rating" indicating the quality or confidence in the results of the study producing the CMF. A higher number of stars indicate a better rating, with five stars representing the best quality of research for the CMF. Each CMF will have an accompanying study along with the countermeasure's impact on crash severity, crash types, and where the countermeasure was deployed (e.g., rural or urban area).

The *Highway Safety Manual* (HSM), published in 2010, provides practitioners with information and tools to consider safety when making decisions concerning the design and operation of roadways. The CMFs used in the HSM are considered the "best of the best" at the time of publication. This tool can help practitioners evaluate alternatives and determine expected impacts on roadway safety. Two chapters in Part C Predictive Method are specific to rural roadways. Chapter 10 provides a methodology to analyze rural two-lane roadways, and chapter 11 is about rural multilane highways. It is important to note the current HSM pertains to only paved roadways.

Estimating Countermeasure Benefits

The Highway Safety Manual and CMF Clearinghouse also provide directions for how to calculate the combined effects of applying multiple safety improvements. In these cases, the CMFs are typically multiplied to estimate the combined effect of independent countermeasures such as adding pedestrian signals and left-turn lane at a signalized intersection. The HSM recommends that practitioners multiply no more than 3 CMFs to estimate the combined effect of multiple safety improvements. Practitioners are cautioned about multiplying CMFs for countermeasures targeting the same crash type, such as using chevrons and widened shoulders at a curve to reduce roadway departure crashes. This practice can overestimate the benefits of combined treatments. In this case, caution and engineering judgment should be exercised.2

While the number of CMFs for newer or more innovative safety improvements is limited, the CMF Clearinghouse can provide local and rural road managers with a good start in compiling benefits and comparing the relative effectiveness of potential improvements.

Local and rural road operators can gain an understanding of safety treatment effectiveness by comparing CMFs for countermeasures that reduce the occurrence of the same crash type. For example, the table below presents a series of safety countermeasures to treat run-off-the-road crashes, a frequent challenge on two-lane local and rural roads, by increasing cost.

Countermeasure
			CMF
			Cost
			ReferenceInstall Advance Curve Warning Signs
			0.7
			Low
			R. Elvik, and T. Vaa, "Handbook of Road Safety Measures," Oxford, United Kingdom, Elsevier (2004).Provide Road Delineation Signing
			0.65
			Low
			A. Montella, "Safety Evaluation of Curve Delineation Improvements An Empirical Bayes Observational Before-After Study," TRB 88th Annual Meeting Compendium of Papers CD-ROM, Washington, D.C., (2009).Install Edge Line Striping
			0.62 to 0.56
			Low to Moderate
			X. Sun and S. Das, "Safety Improvement from Edge Lines on Rural Two-Lane Highways," Louisiana Department of Transportation and Development, Report No. FHWA/LA.11/487, Baton Rouge, LA, (2011).Install Edge Line Rumble Strips
			0.90 to 0.78
			Low to Moderate
			D.J. Torbic, J.M. Hutton, C.D. Bokenkroger, et al. "Guidance for the Design and Application of Shoulder and Centerline Rumble Strips," *Transportation Research Board*, Washington D.C., (2009).Widen Shoulders
			0.98
			Low to Moderate
			K. Haleem, A. Gan, and J. Lu. "Using multivariate adaptive regression splines (MARS) to develop crash modification factors for urban freeway interchange influence areas," *Accident Analysis and Prevention*, Vol. 55, (2013) pp. 12-21.Remove or Shield Roadside Obstacles
			0.62
			Low to Moderate
			P.W. Hovey and M. Chowdhury, "Development of Crash Reduction Factors," 14801(0), Ohio Department of Transport, (2005).Flatten Horizontal Curve
			0.33
			High
			Pitale, J.T., Shankwitz, C., Preston, H., and Barry, M., "Benefit-Cost Analysis of In-Vehicle Technologies and Infrastructgure Modifications as a Means to Prevent Crashes Along Curves and Shoulders," Minnesota Department of Transportation, (2009).Source: Federal Highway Administration, "Crash Modification Factors (CMF) Clearinghouse." Available online at: www.cmfclearinghouse.org.

As presented in this table, the installation of advance curve warning signs has a CMF of 0.70. By applying this treatment to horizontal curves along a two-lane rural road experiencing an average of ten horizontal curve/run-off-the-road crashes per year, one can expect seven horizontal curve/run-off-the-road crashes per year following the implementation of the countermeasure (10 x 0.70 = 7). In other words, crashes can be reduced by 30 percent. Conversely, widening shoulders, a more costly countermeasure, would result in a more modest reduction in crashes. Based on this knowledge, and supplemented with experienced application of CMFs, local and rural road agencies can quickly understand the potential safety benefits from applying a range of safety treatments.

**Crash Modification Factor Example:**

Average Crashes (after CM implementation) − (CMF × Avg. Crashes (Before CM Implementation)) = Crash Reduction

**Adding Advance Warning Signs at Curves**

10 − (0.7 × 10 crashes/year) = 3 crashes per year reduced

Some CMFs apply to the average of all crashes on a roadway segment or intersection, while others may apply to crashes based on severity, such as fatal or injury crashes. This allows agencies to rank the benefits of countermeasures based on the severity of the crashes that can be prevented and to target more severe crashes in a road safety strategy.

Local and rural road owners are sometimes able to fund highly effective, lower-cost projects using local funds more quickly than they can fund more effective but perhaps considerably more costly projects. Reviewing CMFs, along with implementation costs, provides local and rural road owners with an opportunity to understand the benefit/cost (B/C) ratio of different countermeasures before undertaking a more detailed assessment. As shown in Table 1, a local road owner could choose to implement a lower cost option, such as signing or striping, as a first step in improvement while perhaps waiting for State or Federal funding for more costly strategies that may be have greater long-term effectiveness.

Resources

American Association of State Highway and Transportation Officials, *Highway Safety Manual*. Available online at: http://www.highwaysafetymanual.org/

Federal Highway Administration, Introduction to Crash Modification Factors, FHWA-SA-13-015 (Washington, DC: 2013). Available online at: http://safety.fhwa.dot.gov/tools/crf/resources/cmfs/intro.cfm

Federal Highway Administration, "Crash Modification Factors (CMF) Clearinghouse" web page. Available online at: www.cmfclearinghouse.org

Federal Highway Administration, "Crash Modification Factors in Practice" web page. Available at: http://safety.fhwa.dot.gov/tools/crf/resources/cmfs/

F. Gross and K. Yunk, "Using CRFs to Improve Highway Safety," *Public Roads*, May/June (2009)26- 31. Available at https://www.fhwa.dot.gov/publications/publicroads/09june/04.cfm

1 Federal Highway Administration, "Crash Modification Factors (CMF) Clearinghouse" web page. Available online at: www.cmfclearinghouse.org

2 Ibid.

Page last modified on November 24, 2014 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