Frequently Asked Questions

What is the purpose of the CMF Clearinghouse?

The Crash Modification Factors (CMF) Clearinghouse was established to provide transportation professionals:

The purpose of the CMF Clearinghouse is to compile all documented CMFs in a central location. The CMF Clearinghouse provides a searchable database that can be easily queried to identify CMFs to meet user's needs.

The CMF Clearinghouse will be updated on a regular basis to add recently developed and documented CMFs. New CMFs will be identified via a periodic review of published literature. In addition, the CMF Clearinghouse provides a mechanism for transportation professionals to submit documentation of new CMFs to be considered for inclusion.

Educational information on CMFs includes the "About CMFs" page, which summarizes useful information in the form of answers to frequently asked questions. The "Resources" page provides additional information on related trainings and publications.

The inclusion of all CMFs in the CMF Clearinghouse also serves an educational purpose. One important lesson is that reported CMFs have varying quality and applicability to a given users needs.

The CMF Clearinghouse summarizes published information on each CMF, including how it was developed (e.g., study design, sample size, and source of data) and what are its statistical properties (e.g., standard error). Where available, a link is provided to the publication from which the CMF was extracted.

The CMF Clearinghouse reports this information in a standard format to enable users to make educated decisions about the most applicable CMF to their condition. To aid users in assessing the quality of the CMF presented, the CMF Clearinghouse reports a star quality rating. The star quality rating is assigned based upon the standard error of the CMF value, as well as the design, potential biases, data source, and sample size of the study that developed the CMF.

The CMF Clearinghouse also reports whether or not the CMF is included in the Highway Safety Manual. The Highway Safety Manual includes only a subset of CMFs that meet strict inclusion criteria. The CMF Clearinghouse provides the broader context of the larger population of CMFs, from which those included in the Highway Safety Manual were drawn.

The CMFs that are included in the Highway Safety Manual will typically have a higher star quality rating given the strict inclusion criteria. High quality CMFs do not exist for every countermeasure and, therefore, there are many countermeasures for which CMFs do not appear in the Highway Safety Manual. The CMF Clearinghouse includes any documented CMF; i.e., it includes CMFs that do not appear in the HSM either because they did not meet the HSM inclusion criteria or because they were documented after the Manual was completed. As a result, the Clearinghouse includes more CMFs for more countermeasures than the HSM.

Inclusion of a CMF in the CMF Clearinghouse does not constitute an endorsement of the CMF or support for its use. The burden is on the user to determine the most appropriate CMF for their analysis need. This determination should be made based upon the CMFs applicability to their condition (i.e. countermeasure being considered and conditions under which it is implemented) and the quality of the CMF.

The Federal Highway Administration hosted a launch Webinar in December 2009 to announce the new CMF Clearinghouse, provide an overview of CMFs and demonstrate the features of the clearinghouse Web site. Please visit http://fhwa.na3.acrobat.com/p69638484/ to access an archived version of this Webinar.

Download the CMF Clearinghouse Brochure (PDF, 2.3 MB)

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What is a CMF?

A crash modification factor (CMF) is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site.

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The CMF Clearinghouse presents both Crash Modification Factors and Crash Reduction Factors. What's the difference?

The main difference between CRF and CMF is that CRF provides an estimate of the percentage reduction in crashes, while CMF is a multiplicative factor used to compute the expected number of crashes after implementing a given improvement. Both terms are presented in the Clearinghouse because both are widely used in the field of traffic safety.

Mathematically stated, CMF = 1 - (CRF/100). For example, if a particular countermeasure is expected to reduce the number of crashes by 23% (i.e., the CRF is 23), the CMF will be 1 - (23/100) = 0.77. On the other hand, if the treatment is expected to increase the number of crashes by 23% (i.e., the CRF is -23), the CMF will be = 1 - (-23/100) = 1.23.

These reduction estimates might also be expressed as a function. Crash reduction and crash modification factors are constants; crash modification functions allow the factor to vary for different scenarios, such as for different traffic volume scenarios.

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I've seen the term "Accident Modification Factor" (AMF) before. Is that different than a Crash Modification Factor?

Aside from the name, Accident Modification Factor is the same thing as Crash Modification Factor (i.e., AMF of 0.80 = CMF of 0.80). Although the CMF Clearinghouse does not use the term "AMF", there are instances of its use in various areas of the safety field. For example, early drafts of the Highway Safety Manual used the term "AMF", but the decision was made to change the terminology to "CMF" for the final publication.

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How can I apply multiple CMFs?

If multiple countermeasures are implemented at one location, then common practice is to multiply the CMFs to estimate the combined effect of the countermeasures. In fact, there is limited research documenting the combined effect of multiple countermeasures. Although implementing several countermeasures might be more effective than just one, it is unlikely the full effect of each countermeasure would be realized when they are implemented concurrently, particularly if the countermeasures are targeting the same crash type. Therefore, unless the countermeasures act completely independently and target unique crash types, multiplying several CMFs is likely to overestimate the combined effect. The likelihood of overestimation increases with the number of CMFs that are multiplied. Therefore, much caution and engineering judgment should be exercised especially when estimating the combined effect of more than 3 countermeasures at a given location.

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What does the star quality rating mean?

The star rating indicates the quality or confidence in the results of the study producing the CMF. The star rating is based on a scale (1 to 5), where a 5 indicates the highest or best rating. The review process to determine the star rating judges the accuracy and precision as well as the general applicability of the study results. Reviewers considered five categories for each study — study design, sample size, standard error, potential bias, and data source — and judged each CMF according to its performance in each category. Read more detailed information on the star quality rating system.

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How is the star quality rating different from the notations (bold, italics, etc.) in the Highway Safety Manual?

The star rating and the HSM notation are similar but different. Both indicate the same thing, which is a confidence in the CMF based on the quality of the study that produced it. In a rough sense, higher star ratings correspond to a bold face HSM notation and mid-range star ratings correspond to italics and asterisk HSM notations, but there is not a one-to-one comparison laid out between the two systems. The differences exist in the way the CMFs are reviewed to determine their quality.

The HSM review process applies an adjustment factor to the standard error from the study, and then assigns the bold and italic notations based on ranges of the adjusted standard error. The standard error is adjusted based mainly on the quality of the study design. The HSM assigns asterisk (*) or caret (^) notations based on the confidence interval of the CMF, which indicates how accurate the CMF estimate is.

The CMF Clearinghouse review process rates the CMF according to five categories — study design, sample size, standard error, potential bias, and data source — and judged the CMF according to its performance in each category. It assigns a star rating based on the cumulative performance in the five categories. It differs from the HSM process in that it does not attempt to adjust the standard error as the HSM does, and it more explicitly considers criteria such as data source, which examines whether a study used data from just one locality or from multiple locations across the state or nation.

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How can I submit my own CMF for inclusion in the CMF Clearinghouse?

The CMF Clearinghouse welcomes CMF study submissions. Please use the provided form to submit your study, but please be sure to search before submitting a new CRF as it may already be listed. You may either submit a link to a resource already existing on the web (preferred) or upload your own file.

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Are there available trainings related to the application of CMFs?

The National Highway Institute offers training resources on Crash Modification Factors. Please visit the Resources Section to find out more on available trainings.

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How does the CMF Clearinghouse relate to the Highway Safety Manual?

The Crash Modification Factors Clearinghouse is just one of the tools and resources available to help transportation professionals make safety decisions. The Highway Safety Manual, expected to be released in the spring of 2010, will provide practitioners with the best factual information and tools to facilitate roadway design and operational decisions based on explicit consideration on their safety consequences.

The CMF Clearinghouse incorporates information relating to the HSM within this Web site. Users are able to view and search for CMFs included in the HSM. The CMF Clearinghouse includes all of the CMFs listed in the HSM.

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How do you determine statistical significance?

A CMF is determined to be statistically significant if the specified confidence interval of the CMF does not include 1.0, since a value of 1.0 indicates no effect from the countermeasure. For a given CMF and standard error, the confidence interval will depend on the significance level that is used. The two most common significance levels are 0.05 (corresponds to 95% confidence interval) and 0.10 (corresponds to 90% confidence interval).

For the 95% confidence level, the confidence interval is equal to the CMF ± 1.96 * (standard error).
For the 90% confidence level, the confidence interval is equal to the CMF ± 1.64 * (standard error).

Example

The CMF for countermeasure A is 0.80 with a standard error of 0.15. The lower and upper limits of the 95% confidence interval are the following:

Lower limit: 0.80 – 1.96*0.15 = 0.80 – 0.294 = 0.506
Upper limit: 0.80 + 1.96*0.15 = 0.80 + 0.294 = 1.094

Since the 95% confidence interval (0.506, 1.094) includes 1.0, this CMF is not statistically different from 1.0 (at the significance level 0.05, i.e., confidence level 0.95)

On the other hand, if the same CMF had a standard error or 0.09, then the lower and upper limits of the 95% confidence interval will be the following:

Lower limit: 0.80 – 1.96*0.09 = 0.80 – 0.1764 = 0.6236
Upper limit: 0.80 + 1.96*0.09 = 0.80 + 0.1764 = 0.9764

Since the 95% confidence interval (0.6236, 0.9764) does not include 1.0, this CMF is statistically different from 1.0 (at the significance level 0.05, i.e., confidence level 0.95)

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Who uses CMFs and how are they used?

CMFs are used by several groups of transportation professionals for various reasons. The primary user groups include highway safety engineers, traffic engineers, highway designers, transportation planners, transportation researchers, and managers and administrators. CMFs can be used to:

Examples:

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How are CMFs added to the Clearinghouse and what is the process for review?

CMFs for the Clearinghouse are obtained either through 1) a regular examination of presented or published material or 2) studies that are submitted by Clearinghouse users through the web site. All studies that are determined to be eligible for the Clearinghouse (i.e., studies that produce one or more CMFs) are submitted to a review process. Presented and published studies considered for the CRF Clearinghouse will be taken from the following sources:

The review process evaluates each study according to its study design, sample size, standard error, potential bias, and data source and gives a rating of excellent, fair, or poor to each category. These ratings correspond to point values in a scoring system that is used to determine the star quality rating.

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The information contained in the Crash Modification Factors (CMF) Clearinghouse is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in the CMF Clearinghouse. The information contained in the CMF Clearinghouse does not constitute a standard, specification, or regulation, nor is it a substitute for sound engineering judgment.