CMF Update, Summer 2015
The summer 2015 edition of CMF Update is the 11th edition of the Crash Modification Factors Clearinghouse e-newsletter. To subscribe, please visit http://www.cmfclearinghouse.org/newsletter_signup.cfm.
To view archived issues, please visit
The CMF Clearinghouse is continually updated with new CMFs. Below are CMFs recently added to the Clearinghouse database.
Star Quality Rating:
Widen paved shoulder from 4 ft to 6 ft
Crash Modification Factor (CMF) Value: 0.98
Star Quality Rating:
Installation of fixed speed cameras
Crash Modification Factor (CMF) Value: 0.92
Star Quality Rating:
Crash Modification Factor (CMF) Value: 0.77
Star Quality Rating:
CMF Clearinghouse User Guide
The CMF Clearinghouse now provides a guide for new users. The User Guide includes information about crash modification factor (CMF) basics for those unfamiliar with CMFs and guidance on how to conduct searches on the CMF Clearinghouse. It also provides advanced tips and functionality for more experienced users.
The User Guide is organized into the following sections:
- Introduction to Crash Modification Factors: This section provides basic information about what CMFs are and how they are used.
- Introduction to the CMF Clearinghouse: This section informs readers about the purpose and contents of the CMF Clearinghouse.
- Searching for CMFs on the CMF Clearinghouse: This section shows users how to use the search functionality of the Clearinghouse to find CMFs.
- Identifying Appropriate CMFs: This section provides guidance on interpreting search results and selecting the most appropriate CMF for a given situation.
- Information for Advanced Users: This section provides information about CMFs and the Clearinghouse for those with more experience. It includes guidance on downloading extracts of the CMF Clearinghouse database and guidance on developing CMFs.
Tutorial videos will accompany the user guide. They will demonstrate how to use features of the Clearinghouse, such as how to conduct a search and how to filter a search result. Stay tuned for more information coming soon!
When choosing which countermeasure to use in a given area, transportation professionals should consider the countermeasure’s service life – the amount of time the countermeasure is expected to last before needing replacement – and crash severity costs – the economic impact of a crash based on its severity. This information assists in the economic decision making process by allowing the roadway designer to conduct a cost/benefit analysis for implementing the particular improvement over a given period.
Most states compile their own, separate, information on service lives of countermeasures and crash severity costs that they use for economic appraisals. Some of this documentation is made publicly available on State DOT safety websites or published in the Highway Safety Improvement Program manuals, but never before has all available information for states been compiled in one place for all to access. As part of this synthesis project, researchers searched various sources for available resources, identified the relevant information and synthesized it in two databases: one for countermeasure service life and the other for crash severity costs. The data and a user’s guide are available on the CMF Clearinghouse website.
Safety performance functions (SPFs) are one of the fundamental building blocks of the predictive methods in the Highway Safety Manual (HSM). One step in implementing the HSM is calibration of the HSM-provided SPFs or development of jurisdiction-specific SPFs. Many States have already completed this step. This spreadsheet (compiled in July 2014) summarizes the results from ten States that have published their HSM calibration factors and/or State-specific SPFs.
When applying two countermeasures at the same location, why would you ever multiply CMFs from both countermeasures? Wouldn’t you just choose one? Multiplying will give you a much smaller CMF. For example, 0.5x0.5 = 0.25, which seems counterintuitive.
If you are applying two countermeasures which are expected to have independent effects, that is, they will address different crash types, it would be reasonable to estimate their combined effect by multiplying. For example, this could be true for an application of shoulder rumble strips (to decrease run off road crashes) and crosswalk enhancements (to decrease pedestrian crashes) on a segment of road. In your example, each countermeasure has a CMF of 0.5 (presumably for “total crashes”), which means that the countermeasure is expected to decrease the total crashes by half. If it is true that both countermeasures in your example have independent effects, then one countermeasure would reduce the total crashes by half (0.5), and the second countermeasure would further reduce that by half (0.25).
However, in most situations, countermeasures which are applied together at one location are related in terms of which crash type they address. For instance, an agency might apply post-mounted delineators and wider edgelines together at horizontal curves. Both of these countermeasures are intended to improve the delineation of the curve and prevent run off road crashes. In this case, multiplying their CMFs would not be appropriate. If both countermeasures had a CMF of 0.5 for total crashes, then the first countermeasure could be expected to reduce total crashes by half (0.5), but the effectiveness of the second countermeasure would be much more limited, since the first countermeasure has already reduced the type of crash that the second countermeasure is targeting. Thus, conservatively we can use 0.5 as the final CMF, or we can increase it slightly with the assumption that the second countermeasure will still have some effect. A white paper by Gross and Hamidi provides further details on when it is appropriate to multiply and when it is appropriate to use another method to estimate the combined effect.
Further guidance on methods of calculating CMFs for multiple treatments will be developed in the ongoing NCHRP 17-63 project.
This Featured FAQ appears in the Q&A document created from questions asked during the most recent CMF Clearinghouse webinar, Applying or (mis-applying) CMFs: The ins and outs of estimating crash reductions. Visit the webinars page of the Clearinghouse website to access the full list of questions asked during the webinar and their answers and the webinar slides, or to view a recording of the webinar.
The CMF Clearinghouse welcomes CMF study submissions to be included in its searchable database. Please use the provided form at http://cmfclearinghouse.org/pubsubmit.cfm to submit your study. Be sure to search before submitting a new CMF 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. Submissions might include published research studies that are not presented in the Clearinghouse, or state-specific CMFs that were developed as part of the Highway Safety Improvement Program.