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 Advanced Search feature of the website, users are able to view and search 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 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 five categories — study design, sample size, standard error, potential biases, and data source — and judged the CMF according to its performance in each category. It assigns a star rating (one through five) based on the cumulative performance in the five categories. It differs from the HSM process in that it does not attempt to adjust the standard error, but similarly to the HSM it explicitly considers criteria such as data source, which examines whether a study used data from just one locality or from multiple locations across the state or nation, among others.
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
For more information, please visit http://www.highwaysafetymanual.org/
How CMF Clearinghouse assigned star ratings to HSM CMFs
The CMF Clearinghouse team 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 Method Correction Factor (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?
|0 – 0.05||Yes||5|
|0.05 – 0.2||Yes||4|
|0.2 – 0.4||Yes||3|
|0 – 0.4||No||3|
|0.4 – 0.5||No||2|