U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: FHWA-HRT-11-001 Date: November/December 2010|
Publication Number: FHWA-HRT-11-001
Issue No: Vol. 74 No. 3
Date: November/December 2010
FHWA has developed an accessible Web-based, one-stop resource for information on crash modification factors.
|The CMF Clearinghouse contains more than 2,500 crash modification factors for 700+ countermeasures. Installing a sidewalk (with a CMF of 0.26) like the one shown here improves safety by removing pedestrians from the roadway. State surveys indicate that this can reduce crashes by 74.|
The Federal Highway Administration (FHWA), State departments of transportation (DOTs), and other stakeholders continue to make progress in reducing highway fatalities across the Nation, with deaths per vehicle mile traveled (VMT) falling every year except 1 over the last 15 years. In 1994 and 1995, for instance, there were 1.73 fatalities per 100 million VMT, but that number dropped to 1.25 deaths per 100 VMT in 2008, according to the National Highway Traffic Safety Administration.
Continuing to achieve further reductions in traffic fatalities will require even more effective, data-driven investment decisions. Practitioners now have many resources and tools available to help them identify potential safety improvements and decide which ones to implement. One such resource is crash modification factors (CMFs), multiplicative factors used to compute the expected number of crashes that might occur after implementing a given countermeasure at a specific site. The concept is not new, as efforts can be traced back to the 1970s to develop tabular summaries of accident reduction factors, or ARFs, as they were called at that time. Over the years, researchers have developed thousands of CMFs to estimate the expected safety improvement associated with implementation of various countermeasures.
The CMF represents a valuable piece of information for safety professionals. A CMF of less than 1.00 indicates an expected decrease in the number of crashes, while a CMF greater than 1.00 indicates an expected increase in crashes. For example, imagine that an intersection is experiencing 20 angle crashes and 40 rear-end crashes per year. If a DOT implements automated red light running enforcement cameras, which have a CMF of 0.67 for angle crashes, the agency might expect to see 13 angle crashes (20 x 0.67 = 13) per year after implementation. If the same countermeasure also has a CMF of 1.45 for rear-end crashes, the DOT might expect to see 58 rear-end crashes (40 x 1.45 = 58) per year. By performing these calculations, engineers can weigh the relative costs and benefits of installing various countermeasures and inform decisionmakers about the solution(s) most likely to improve overall safety at a given location.
Research continuously identifies new CMFs, but they are useful only if easily available to practitioners. Recognizing the growing need for a centralized location to store and provide easy access to the CMFs, FHWA recently launched the Web-based Crash Modification Factors Clearinghouse (www.CMFClearinghouse.org). As of August 2010, the clearinghouse provides access to more than 2,500 CMFs for over 700 countermeasures, as well as guidance to help transportation professionals use CMFs to improve their decisions about road safety. The site also features information on training and cost-benefit analyses.
"The Crash Modification Factors Clearinghouse provides an easy way for practitioners to use the latest knowledge as they make important safety improvement decisions on their roadways," says FHWA Executive Director Jeff Paniati. "It also provides links to other important safety resources, such as the new [American Association of State Highway and Transportation Officials (AASHTO)] Highway Safety Manual."
|The home page of the CMF Clearinghouse features recently added CMFs.|
In November 2008, FHWA began developing the CMF Clearinghouse. The agency worked with a variety of potential users, such as State DOT personnel and local engineers, to develop the content, design, and functionality of the Web site. Based on this indepth user feedback, FHWA structured the clearinghouse to include several key features. First, the site includes a rating system to inform users of the reliability of CMFs. Second, the site lists both CMFs and crash reduction factors (or CRFs), which are estimates of the percentage reduction in crashes. Plus, the site is home to assorted educational materials that are updated regularly. The clearinghouse also coordinates closely with information covered in AASHTO's Highway Safety Manual, which is a key document practitioners use to facilitate roadway design and operational decisions based on explicit consideration of their safety consequences.
CMFs and CRFs
Crash modification factors (CMFs) and crash reduction factors (CRFs) are both terms used to express the effectiveness of countermeasures installed to improve highway safety. The main difference is that CMFs are used to compute the expected number of crashes after implementing certain improvements, while CRFs provide estimates of the percentage reduction in crashes. A CRF of 46, for example, represents a 46 percent reduction in crashes at a particular site. CMFs and CRFs are related by a simple mathematical function: CMF = 1 - (CRF/100). Using the previous example, a CRF of 46 would yield a CMF of 0.54. Because both terms are widely used in the field of traffic safety, both are available in the clearinghouse.
In September 2007, FHWA published the Desktop Reference for Crash Reduction Factors (FHWA-SA-07-015), the first comprehensive resource on countermeasure effectiveness. FHWA then updated the resource the following year (FHWA-SA-08-011) to reflect the latest research. In addition to including CMFs, FHWA's CMF Clearinghouse includes CRFs because of their widespread use and displays them side by side on the basic search results page and each CMF details page.
The initial collection of CMFs in the clearinghouse came from several sources: research conducted to develop AASHTO's Highway Safety Manual, FHWA's Desktop Reference for Crash Reduction Factors, and studies identified at the 2009 Transportation Research Board annual meeting. After drawing on existing compilations of CMFs to populate the clearinghouse upfront, FHWA now updates the site quarterly as new CMFs become available.
|Studies have shown that providing continuous milled-in shoulder rumble strips (with a CMF of 0.21) like those shown here can lead to a 79 percent reduction in crashes.|
FHWA identifies additional CMFs for the updates through literature searches and user submissions, and then reviews all potential new CMFs to determine their applicability for the clearinghouse. The review process has two parts. First, a preliminary review identifies and records key information about studies with potential relevance to the clearinghouse. This step records information such as the study title and publication date, countermeasures investigated, study methodology, sample size, and locations used for data collection. Second, a critical review then evaluates each CMF and determines an appropriate quality rating. After FHWA assigns a rating, the CMF goes live on the clearinghouse Web site.
CMFs in the Highway Safety Manual
The Highway Safety Manual provides tools for highway safety practitioners to develop effective roadway safety management programs and predict estimates of crash frequency and severity. The Highway Safety Manual also provides a catalog of crash modification factors (CMFs) for a variety of facility types. Practitioners use CMFs to estimate the change in the number of crashes as a result of implementing a particular countermeasure. CMFs also can be used to select countermeasures and conduct economic appraisals as part of the roadway safety management process or to estimate the expected average crash frequency of a proposed project or design alternative using the predictive methods.
The CMF Clearinghouse includes all documented CMFs, which can vary widely in quality and reliability depending on the study design, number of sites included in the analysis, and other factors. For this reason, the potential users who were consulted requested that the clearinghouse include a system to indicate the dependability of each CMF. In response, FHWA developed a quality rating system utilizing stars -- the more stars, the better the quality of the CMF.
|AASHTO published the first edition of the Highway Safety Manual (shown here) in 2010.|
FHWA bases the quality rating on a CMF's performance (that is, the quality of the study that developed the CMF) in five categories: study design, sample size, standard error, potential bias, and data source. The performance in each category is rated as excellent, fair, or poor. For example, a study that employed a statistically rigorous design with a reference group, such as empirical Bayes (a method by which predicted crashes are compared to actual crashes to determine the safety effect of the countermeasure), would receive a rating of excellent for study design. If the study employed a simple before/after design, it would receive a lower rating relative to study design. However, study design is only one category. If the study also had a large sample size or widespread data source, it would receive high scores for those categories. Scores across all five categories are combined to produce the star quality rating for the CMF.
The quality rating system applies criteria that are intended to be as objective as possible, but ratings still entail a degree of subjectivity and judgment. "Users of the clearinghouse should take into account all the information presented for a CMF and should not substitute the star quality rating for sound engineering judgment," says Ray Krammes, technical director, FHWA Office of Safety Research and Development.
Although the star quality rating provides users with an indication of the general reliability of the CMF, one of the best ways to ensure CMFs used by practitioners match the local conditions as closely as possible is for State and local agencies to develop CMFs using local data.
Coordination with the Highway Safety Manual was an essential factor in development of the CMF Clearinghouse. In fact, to be consistent with the manual, the predominant term in the clearinghouse is "crash modification factor" rather than "crash reduction factor," which FHWA used in previous related documents.
The CMFs in the manual meet strict inclusion criteria, as described in Transportation Research Circular E-C142 Methodology for the Development and Inclusion of Crash Modification Factors in the First Edition of the Highway Safety Manual, whereas FHWA's clearinghouse provides a comprehensive list of all available CMFs. To help users quickly determine whether a CMF is included in the Highway Safety Manual, the clearinghouse enables them to search for CMFs in the manual. 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. In addition, there are many CMFs that were documented after the Highway Safety Manual was developed.
"The review process for the CMF Clearinghouse has the added benefit of setting the stage for the next edition of the Highway Safety Manual," says Priscilla Tobias, State safety engineer with the Illinois Department of Transportation and chairperson of the AASHTO Subcommittee on Highway Safety Management's Technical Safety Publication Oversight and Coordination Task Force. "Highway Safety Manual reviewers will be able to use the information compiled for the clearinghouse to review CMFs and determine whether they meet the manual's inclusion criteria."
|Studies have shown that converting a two-way, stop-controlled intersection to a roundabout (with a CMF of 0.56), such as the one shown here, can reduce crashes by 44 percent.|
Launched in December 2009, the clearinghouse hosted more than 6,300 visits in just its first 8 months. Traffic engineers and other professionals now use the clearinghouse to answer basic questions they face every day, such as, "What is the best safety countermeasure to use?" To find applicable CMFs, users can conduct quick keyword searches from the home page or narrow their queries by countermeasure, crash type, crash severity, and roadway type. The Web site also has an advanced search feature that enables users to search by more detailed parameters, such as intersection type, traffic control, area type, and more.
State DOTs are promoting the clearinghouse to transportation officials who are conducting benefit-cost analyses. The Iowa Department of Transportation (Iowa DOT), for example, uses the clearinghouse as a resource for local governments that are applying for site-specific safety funding through the State's Traffic Safety Improvement Program. When seeking grants for either new construction or improvement of traffic safety and operations at a specific site or corridor with a crash history, applicants must include benefit-cost analyses. Using worksheets provided by Iowa DOT, local transportation officials calculate the benefit-cost ratio for each potential improvement. Iowa DOT directs applicants to the CMF Clearinghouse as a starting point for these analyses and instructs them to use the CRFs there. "We especially like that in most cases there are factors that are specific to certain crash types and severities," says Tim Simodynes, a safety engineer with Iowa DOT.
The Washington State Department of Transportation (WSDOT) also is using the clearinghouse to provide guidance on benefit-cost analyses. With the complete list of CMFs included in the clearinghouse as a starting point, WSDOT is developing a tailored list of CMFs approved for use by the department and will distribute that list to potential applicants for local safety funding.
"Providing our agency with CMFs from the clearinghouse will enable WSDOT to better scope and prioritize our projects," says Matt Neeley, intelligent transportation systems research and planning engineer with WSDOT. "In the past, we haven't had as much information as the clearinghouse provides."
Another purpose of the clearinghouse is to educate transportation professionals about the application of CMFs. The Web site includes an overview of CMFs and a glossary of related terms. Users can read a list of frequently asked questions that address issues such as the difference between CMFs and CRFs, and how to apply multiple CMFs at one location. The site also includes a comprehensive resources section with links to CMF-related publications, countermeasure selection tools, and how to sign up for the CMF Update, the clearinghouse's e-newsletter.
The clearinghouse also directs users to two Web-based training courses dealing with CRFs available through the National Highway Institute (NHI): Application of Crash Reduction Factors (FHWA-NHI-380093) and Science of Crash Reduction Factors (FHWA-NHI-380094). The former provides hands-on experience with safety diagnosis and application of CRFs to compare the effectiveness of countermeasures.
According to Kathy DesRoches, director of workforce development at Manchester Community College, New Hampshire hosted a session of the course Application of Crash Reduction Factors in March 2009 as a prerequisite for individuals planning to attend a road safety audit (RSA) course. "By taking the CRF course first," DesRoches says, "participants were better equipped to conduct an RSA because they were more informed about low-cost countermeasures and linking crash patterns with specific countermeasures."
Now that the clearinghouse is up and running, FHWA will continue to update the database with new CMFs as they become available. FHWA invites users to submit feedback regarding the site's design and content. Past feedback has resulted in improvements to the search mechanism, better explanations of site items, and a clearer layout.
Top Ten Most Viewed CMFs
The following represent the top ten CMFs that received the most views by visitors to the CMF Clearinghouse between December 1, 2009, and August 15, 2010.
In addition, FHWA encourages transportation professionals to make efforts to develop CMFs within their own agencies by conducting safety evaluation studies using data from their jurisdictions. These evaluation studies can be submitted for possible inclusion in the clearinghouse. By adding to the library of documented CMFs, practitioners can share their own research with other States and assist their colleagues in making data-driven decisions that can help save lives.
|One of the goals of the CMF Clearinghouse is to educate practitioners on the proper application of CMFs. Shown here is the opening slide in the NHI course Application of Crash Reduction Factors (FHWANHI- 380093).|
Katy Jones is the manager for research information and education programs at the University of North Carolina Highway Safety Research Center. She has a B.A. degree in journalism and mass communication from the University of North Carolina at Chapel Hill. She also serves as the marketing manager for the Pedestrian and Bicycle Information Center.
Karen Yunk, P.E., is the Highway Safety Improvement Program implementation manager with FHWA's Office of Safety Programs. She has an M.S. degree in civil engineering from Rutgers, the State University of New Jersey.
Daniel Carter, P.E., is an engineering research associate at the University of North Carolina Highway Safety Research Center, where he handles data collection and analysis for safety research projects. He received B.S. and M.S. degrees in civil engineering (transportation) from North Carolina State University.
For more information, contact Katy Jones at 919-843-7007 or firstname.lastname@example.org, Karen Yunk at 609-637-4207 or email@example.com, or Daniel Carter at 919-962-8720 or firstname.lastname@example.org. To sign up for news about the CMF Clearinghouse, please visit www.CMFClearinghouse.org/signup.