Crash Modification Factors in the Highway Safety Manual: Resources for Evaluation (2023) / Chapter Skim
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Appendix B CMF User Preferences and Practices
Appendix B CMF User Preferences and Practices
Pages 47-96
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Appendix B CMF User Preferences and Practices B-1
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B-4 1.1 Selecting and Applying CMFs ........................................................................................................ B-4 1.2 CMF Quality ...................................................................................................................................
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Responses to Question 7 by Position Type.............................................................................. B-22 Figure 9.
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at the site of interest. This was true for all subgroups of individual position types, except researcher/teachers, where most people applied CMFs to expected crashes produced from safety performance functions.
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. However, the answers by individual position type were divided, with traffic safety engineers, traffic engineers, and researchers/teachers largely preferring that the HSM Part D not contain CMFs, and highway designers, transportation planners, and agency administrators preferring that the HSM continue in the same format as the first edition by including CMFs in Part D
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The following text was used to advertise the questionnaire: The project team for NCHRP 17-72, Update of Crash Modification Factors for the Highway Safety Manual, is conducting a survey of road safety professionals. The goal of NCHRP 17-72 is to identify which CMFs should be eligible for the Highway Safety Manual and how they should be presented.
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Table 1. Questionnaire Participants by Position Position Type Number of Responses Percent of Responses Traffic safety engineering 51 36% Traffic engineering 37 26% Research and/or teaching 22 16% Highway design 10 7% Transportation planning 6 4% Other 6 4% Other engineering 5 4% Agency administration 4 3% Agency State departments of transportation and private consulting firms accounted for more than half the participants (Table 2)
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. The states represented in the questionnaire responses are listed below: • AL • MD • OR • AR • ME • PA • AZ • MI • SD • CO • MN • TN • CT • MO • TX • FL • MS • UT • GA • MT • VA • IA • NC • VT • IL • NH • WA • IN • NJ • Washington, D.C.
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Participants were allowed to choose more than one answer or write in their own response. • CMFs for total crashes • CMFs for specific crash types • CMFs for specific crash severities • I don't use CMFs When responses are viewed as a group for the entire audience, the majority of participants (60%)
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. The highest percentage of responses for most position types was for those selecting all three options (CMFs for total crashes, CMFs for specific crash types, and CMFs for specific crash severities)
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Traffic Safety Engineering Traffic Engineering Research and/or Teaching Highway Design B-11
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Agency Administration Transportation Planning Other Engineering Other Figure 2. Responses to Question 4 by Position Type B-12
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• Historic crash data for the site (e.g., the past 3 to 5 years of crashes) • Expected number of crashes estimated using safety performance functions (SPFs)
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• Depends on available data; have encountered all 3 scenarios • Both historic crash data & SPFs • Mixture of the three The responses from most individual position types mirrored that of the entire group – that people were applying CMFs to modify the historic crash data (Figure 4)
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Traffic Safety Engineering Traffic Engineering Research and/or Teaching Highway Design B-15
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) : The majority of participants apply CMFs to modify historic crash values (the past 3 to 5 years of data)
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The responses by individual position type showed that traffic safety engineers, traffic engineers, researchers/teachers, and highway designs predominantly also used CMF quality indications either always or most of the time (Figure 6)
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Research and/or Teaching Highway Design Agency Administration Transportation Planning Other Engineering Other Figure 6. Question 6 Responses by Position Type Takeaway point from Question 6 (how often do you use information about a CMF's quality or reliability?
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The single select format forced participants to select only one answer. Presumably the participants selected the answer that was the dominant reason for them to use CMF quality information.
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• I will evaluate CMFs based on their origin by geography and agency responsible for development Responses by type of position are shown below in Figure 8. For each type of position except Other and Other Engineering, the top reason for using CMF quality information was to prioritize CMFs.
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engineers, highway designers, and agency administrators each had a small portion of responses that said they did not use information about CMF quality. Traffic Safety Engineering Traffic Engineering Research and/or Teaching Highway Design B-21
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For most of the individual position types, this was also the top answer.
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The intention of question #8 was to determine whether CMF users would consider it a negative thing to have quality criteria that are different for these CMFs that address rare crash types. The answer choices were as follows.
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. Traffic Safety Engineering Traffic Engineering Research and/or Teaching Highway Design B-24
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) : The majority of participants said there should be different judging criteria for CMFs for rare crash types.
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• All available (published) CMFs for each specific condition (crash type, crash severity, site characteristics)
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All subgroups by individual position type indicated that the majority of participants would want to see all available CMFs for each condition. Those from traffic safety engineering, traffic engineering, and agency administration had the highest percentages of respondents that wanted to see only one CMF presented; at 22%, 24%, and 25%, respectively.
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Research and/or Teaching Highway Design Agency Administration Transportation Planning Other Engineering Other Figure 12. Responses to Question #9 by Position Type B-28
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This was true for the combined group of all participants and for all of the individual position types. Written responses generally indicated a preference for some kind of curated directory of CMFs, where there would be an indication of a recommended or practice ready CMF while still listing all other available CMFs.
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The responses by individual position type showed a strong difference between two groups. Traffic safety engineers, traffic engineers, and researchers/teachers largely preferred that the HSM did not contain CMFs but provided the "stamp of approval" to CMFs listed on the CMF Clearinghouse.
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Traffic Safety Engineering Traffic Engineering B-31
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Research and/or Teaching Highway Design B-32
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Agency Administration Transportation Planning B-33
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. However, the answers by individual position type were divided, with traffic safety engineers, traffic engineers, and researchers/teachers largely preferring that the HSM did not contain CMFs, and highway designers, transportation planners, and agency administrators preferring that the HSM continue in the same format as the first edition by including CMFs in Part D
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It has been recognized in discussions among TRB and AASHTO groups that Part D could have guidance related to applying CMFs appropriately (for a practitioner audience) , developing CMFs (for a researcher audience)
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Most position types also had at least 50% of respondents that selected "guidance on how to develop CMFs" as well. The exceptions were transportation planning, other engineering, and other, to whom there was less priority on developing CMFs.
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Research and/or Teaching Highway Design Agency Administration Transportation Planning B-37
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5. PARTICIPANT RESPONSES TO QUESTIONNAIRE: CMF SCENARIOS One concern that is commonly raised in some discussions among TRB and AASHTO committees is whether those who use CMF resources (either the HSM or CMF Clearinghouse)
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Responses by individual position type (Figure 18) did not reveal any trends that differed significantly from the overall group response breakdown.
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Traffic Safety Engineering Traffic Engineering Research and/or Teaching Highway Design Agency Administration Transportation Planning Other Engineering Other B-40
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Most of the incorrect responses were due to the application of the specific crash type CMF to the total crashes of the site. 5.2 Question #13 CMF Selection Question The objective of this question was to determine if the participant knew how to select a CMF correctly.
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Responses by individual position type (Figure 18) did not reveal any trends that differed significantly from the overall group response breakdown.
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: Almost all participants selected the correct CMF that should be used in the scenario.
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For this reason, the research team convened a focus group consisting of traffic safety practitioners who were experienced CMF users. The focus group allowed the opportunity for group discussion and more in-depth responses to issues related to CMFs and the 2nd edition of the HSM.
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? If yes, do you understand what the HSM adjusted standard error is?
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. We go off the star ratings, we typically only use CMFs with a star rating with 3 or above and then try to find something close (in terms of site conditions)
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I've seen a case where someone took a total crash CMF and applied it to only specific severe crash type. Is standard error something you use (or know how to use)
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For example, we wouldn't use a CMF for continuous freeway lighting if we're dealing with lighting an intersection. State 7: Basically the same, how well the study matches the conditions and the target crash type.
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State 8: We do use the CMF Clearinghouse more because it's more up to date. State 7: CMF Clearinghouse is more transparent and more in it than could be in printed manual.
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One of the options is to have the CMFs on the Clearinghouse but some of them will meet criteria for the HSM second edition. If that's the way things go, we'll have to decide how to indicate which ones met HSM approval.
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