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Local and Rural Road Safety Briefing Sheets

Crash Modification Factors (CMFs)

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. 

CMFs for Selected Run-Off-The-Road Crash Countermeasures

Countermeasure CMF Cost Reference

Install Advance 

Curve Warning Signs

0.70 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, DC: TRB, 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: 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. NCHRP Report 641: 

Guidance for the Design and Application of Shoulder and Centerline 

Rumble Strips, National Cooperative Highway Research Program 

(Washington DC: TRB, 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 55 

(2013): 12-21.
Countermeasure CMF Cost Reference

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 Transportation, (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.

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 the table above, 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. 

Crash Modification Factor Example:

Average Crashes (after CM implementation) – (CMF x Avg. Crashes (Before CM Implementation)) = Crash Reduction

Adding Advance Warning Signs at Curves

10 – (0.7 x 10 crashes/year) = 3 crashes per year reduced
Resources

American Association of State Highway and Transportation Officials, Highway Safety Manual. 

Available at: http://www.highwaysafetymanual.org/

Federal Highway Administration, Introduction to Crash Modification Factors, FHWA-SA-13-015 

(Washington, DC: 2013).

Available at: http://safety.fhwa.dot.gov/tools/crf/resources/cmfs/intro.cfm

Federal Highway Administration, “Crash Modification Factors (CMF) Clearinghouse” web page. 

Available 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: http://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.

FHWA-SA-14-083