Establishing crash modification factors and their use.

Gayah, Vikash V.; Donnell, Eric T. · 2014 · ROSA P / Mid-Atlantic Universities Transportation Center

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Summary

This report addresses the integration of Crash Modification Factors (CMFs) into the safety management processes of the Pennsylvania Department of Transportation (PennDOT). CMFs, a critical component of the AASHTO Highway Safety Manual, estimate the change in expected crash frequency at a site when a specific safety countermeasure is implemented. The project was motivated by PennDOT’s need to standardize the use of these factors, ensuring that engineers apply high-quality, statistically valid CMFs consistent with national guidelines. The primary objectives were to assemble a list of CMFs appropriate for Pennsylvania roadways and to provide clear guidelines for their implementation. To achieve these objectives, the authors developed two primary products: the *Pennsylvania CMF Guide* and a training presentation titled *What are CMFs and how do you use them?*. The guidebook was created by reviewing literature from sources such as the FHWA CMF Clearinghouse, the AASHTO Highway Safety Manual, and recent research. The authors filtered these sources using the FHWA’s star quality rating system, which evaluates CMFs based on study design, sample size, standard error, potential bias, and data source. Only CMFs with a rating of three stars or higher were deemed "high-quality" and recommended for use in Pennsylvania, resulting in a curated list of 2,450 CMFs organized into 19 categories, such as access management, intersection geometry, and speed management. Each entry in the guide includes the point estimate, standard error, quality rating, and specific conditions for applicability, such as area type and crash severity. The report details the methodology for applying these CMFs, emphasizing the importance of accounting for statistical uncertainty. It explains that CMFs are point estimates subject to error, which must be addressed using confidence intervals. The training materials demonstrate how to calculate these intervals to determine if a countermeasure provides a statistically significant safety benefit (confidence interval strictly less than one) or disbenefit (strictly greater than one). The guide also provides procedures for applying multiple CMFs simultaneously, distinguishing between scenarios where CMFs affect different crash types versus the same crash types. For independent crash types, the report provides a specific formula to aggregate confidence intervals, noting that simple addition of bounds overestimates variation. The significance of this work lies in its provision of a standardized, evidence-based tool for transportation engineers in Pennsylvania. By restricting recommendations to high-quality CMFs and providing explicit instructions on handling statistical uncertainty and multiple applications, the report aims to improve the accuracy of safety performance predictions. The accompanying training presentation ensures that practitioners can correctly interpret CMFs, select appropriate factors from the guide, and compare alternative countermeasures based on expected safety outcomes. This framework supports more rigorous safety management and better-informed infrastructure investment decisions.

Key finding

The project successfully compiled and validated 2,450 high-quality Crash Modification Factors for Pennsylvania roadways and developed a standardized guidebook and training curriculum to ensure their correct statistical application in safety analysis.

Methodology

dataset

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• incidence prevalence
• induced exposure

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Applied Guidance: countermeasure evaluation
• Empirical Findings: crash risk outcomes