National Academies Press: OpenBook

Guidelines for the Development and Application of Crash Modification Factors (2022)

Chapter: Appendix E - User Guide for the CMF Combination Tool Spreadsheet

« Previous: Appendix D - User Guide for CMF Regression Software
Page 335
Suggested Citation:"Appendix E - User Guide for the CMF Combination Tool Spreadsheet." National Academies of Sciences, Engineering, and Medicine. 2022. Guidelines for the Development and Application of Crash Modification Factors. Washington, DC: The National Academies Press. doi: 10.17226/26408.
×
Page 335
Page 336
Suggested Citation:"Appendix E - User Guide for the CMF Combination Tool Spreadsheet." National Academies of Sciences, Engineering, and Medicine. 2022. Guidelines for the Development and Application of Crash Modification Factors. Washington, DC: The National Academies Press. doi: 10.17226/26408.
×
Page 336
Page 337
Suggested Citation:"Appendix E - User Guide for the CMF Combination Tool Spreadsheet." National Academies of Sciences, Engineering, and Medicine. 2022. Guidelines for the Development and Application of Crash Modification Factors. Washington, DC: The National Academies Press. doi: 10.17226/26408.
×
Page 337
Page 338
Suggested Citation:"Appendix E - User Guide for the CMF Combination Tool Spreadsheet." National Academies of Sciences, Engineering, and Medicine. 2022. Guidelines for the Development and Application of Crash Modification Factors. Washington, DC: The National Academies Press. doi: 10.17226/26408.
×
Page 338

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

E-1   Contents E-1 Introduction E-1 Using the Tool E-2 Example Images Introduction The CMF Combination Tool is a Microsoft Excel spreadsheet implementation of the homo- geneity testing procedure developed under NCHRP Project 17-63: Guidelines for the Develop- ment and Application of Crash Modification Factors. This tool can be used to test two or more CMFs for the same treatment to determine whether they are similar enough to be combined into a single CMF value. The CMFs must also address the same crash type (e.g., total crashes, rear end crashes). If the CMFs are able to be combined, the tool produces a value for the combined CMF. It also gives cautions for the use of the combined value. This tool is not intended for combining CMFs for different treatments. See the Examples below. • Example 1. You have two CMFs that estimate the effect on total crashes of installing wider edgelines. You want to know if they are similar enough to be combined into a single value to be used for an upcoming project to widen edgelines. This tool is intended for this use. • Example 2. You have two CMFs, one for the effect of wider edgelines and one for the effect of rumble strips. You want to calculate a single combined CMF that will provide an estimate of the effect on total crashes if both treatments were installed together. This tool is not for this use. However, guidelines for this purpose are provided under NCHRP Project 17-63. Using the Tool To use the tool, the analyst inputs two or more CMFs and their standard errors (both required) and receives a calculated combined CMF. There must be at least two CMFs entered, but no more than 100 CMFs. Figure E1 shows the input screen from the Main tab of the tool. The top middle portion of the sheet is used to display the resulting combined CMF and caution messages. The top right portion of the screen is used to display a summary of the CMFs entered by the user, including the number of CMFs, the range of the CMF values, and the range of the standard errors. The colored area below these areas is for user input. A P P E N D I X E User Guide for the CMF Combination Tool Spreadsheet

E-2 Guidelines for the Development and Application of Crash Modification Factors The caution messages inform the user about certain conditions that might apply to the results. • Caution 1 alerts the user if the combined CMF is greater than 1.0, indicating that the treat- ment may cause an increase in crashes. • Caution 2 alerts the user if the combined CMF has a standard error that is large enough to put the reliability of the result in question. • Caution 3 alerts the user if more than half of the CMFs have a statistical weight less than 4.0, which calls into question the reliability of the resulting combined CMF. Example Images The screenshots below provide a series of examples showing the tool being used to compare and potentially combine two CMFs. Figure E2 is an example of two CMFs being combined suc- cessfully. Figure E3 through Figure E6 show examples of CMFs that should not be combined or for which the combined result should be used with caution. Figure E1. Main screen of CMF Combination Tool. Figure E2. Example of two CMFs that were combined into a single value.

User Guide for the CMF Combination Tool Spreadsheet E-3   Figure E3. Example of two CMFs that should not be combined because the statistical variability was too large. Figure E4. Example of two CMFs that have a combined result that indicates that the treatment might increase crashes.

E-4 Guidelines for the Development and Application of Crash Modification Factors Figure E5. Example of two CMFs that have a combined result that should be used with caution for producing quantitative estimates of crash frequency reduction because the result may be statistically unreliable due to a large standard error. Figure E6. Example of four CMFs that have a combined result that should be used with caution for producing quantitative estimates of crash frequency reduction because the result may be statistically unreliable due to low statistical weights.

Next: Appendix F - Enhancing Future CMF Research »
Guidelines for the Development and Application of Crash Modification Factors Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Crash modification factors (CMF) provide transportation professionals with the kind of quantitative information they need to make decisions on where best to invest limited safety funds.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 991: Guidelines for the Development and Application of Crash Modification Factors describes a procedure for estimating the effect of a proposed treatment on a site of interest.

Supplemental to the report are a CMF regression tool, a CMF combination tool, a slide summary, and an implementation memo.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!