Relationship 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 (HSM), released in 2010, provides practitioners with the best factual information and tools to facilitate roadway design and operational decisions based on explicit consideration of their safety consequences. One of these tools is the inclusion of crash modification factors, which can be used to support an agency's roadway safety management process or as input to the safety prediction methods.

While the HSM provides only the best available research-based CMFs, the CMF Clearinghouse is a comprehensive listing of available CMFs. The CMF Clearinghouse includes all of the CMFs listed in the HSM. Using the filters on the Search Results page, users are able to view and filter for CMFs included in the HSM.

Both the HSM and the CMF Clearinghouse conducted a review process of CMFs and assigned a "confidence" in the CMF based on the quality of the study that produced it. The HSM review process applies an adjustment factor to the study's CMF (to correct for regression-to-the-mean and traffic volume bias) and a Method Correction Factor (MCF) to the study's standard error (to correct for the study design and method selected, sample size, confounding factors, and other study characteristic documented during the critical review of each study documentation). The adjusted CMFs and standard errors are assigned the bold and italic notations based on ranges of the adjusted standard error. The HSM assigns asterisk (*) or caret (^) notations based on the confidence interval created by the CMF ± two times the standard error, and when this interval includes CMF value of 1.0.

The CMF Clearinghouse review process rates the CMF according to its performance in the various factors (including multiple subcategories within each factor) – study design, sample size, statistical methodology, and statistical significance, amongst others. It assigns a star rating (one through five) based upon the cumulative performance across various factors (with different factors and sub-categories considered for CMFs developed from before-after, cross-sectional, meta-analysis, and meta-regression studies). It differs from the HSM process in that it does not attempt to adjust the standard error, but a MCF is recorded for each CMF.

Complementing the CMF Clearinghouse, the HSM is an effective resource that can be used to quantify and predict the safety performance of the variety of elements considered in road planning, design, maintenance, construction, and operation. Features of the HSM include:

  • Procedures to estimate the crash frequency and severity of sites with or without crash history
  • Procedures to estimate the effect of a treatment using safety performance functions for the facility type
  • Analytical methods for evaluating the safety effect of implemented treatments

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How CMF Clearinghouse assigned star ratings to HSM CMFs

The CMF Clearinghouse team initially assigned star ratings to CMFs from the Highway Safety Manual (HSM) using the adjusted standard error (ASE) that is listed in the HSM and HSM Knowledge Document. The ASE is a value that was determined by the HSM reviewers by multiplying the standard error reported in the study by a MCF, which is a value that is lower for studies of good quality and higher for studies that have quality or bias issues. The resulting ASE is the value used by the HSM to indicate the quality or reliability of a CMF.

The CMF Clearinghouse method of assigning star ratings also took into account whether the CMF was significant within a 95% confidence interval. A CMF was established to be significant if the confidence interval did not include 1.0. As shown in the table below, star ratings were assigned to HSM CMFs based on the value of the ASE and whether the CMF was significant.

Adjusted Standard Error Significant?
(95% conf)
Star Rating
0 – 0.05 Yes 5
0.05 – 0.2 Yes 4
0.2 – 0.4 Yes 3
> 0.4 Yes 2
0 – 0.4 No 3
0.4 – 0.5 No 2
> 0.5 No 1

However, with the update of the CMF Clearinghouse rating system in February 2021, all CMFs from the first edition of the HSM were re-rated based on the CMF rating system developed by NCHRP 17-72. The CMF star rating comparison document (found at HTTP://WWW.CMFCLEARINGHOUSE.ORG/SQR.CFM) lists both the initial star ratings for the HSM CMFs (based on the value of the ASE and whether the CMF was significant) and the updated star ratings (based on the updated CMF Clearinghouse review process).