The National Academies

NCHRP 17-78 [Active]

Understanding and Communicating Reliability of Crash Prediction Models

  Project Data
Funds: $300,000
Staff Responsibility: Mark S. Bush
Research Agency: University of North Carolina - Chapel Hill
Principal Investigator: Dr. Raghavan Srinivasan
Effective Date: 9/20/2016
Completion Date: 3/19/2019


The current Highway Safety Manual (HSM), published in 2010, provides models for predicting crashes for several common roadway facilities and methods for calibrating the models and some qualitative guidance on model reliability. However, the manual does not include methods to consistently convey model reliability. Transparency and credibility are essential to any form of analysis being performed for the public welfare. During initial implementation of the HSM, model results have been generated and often utilized to make major capital improvement decisions without fully understanding and communicating the accuracy of the model results, which can erode the credibility of this new and rapidly growing field. The state of the art of safety analysis has progressed and more has been learned about the impact on accuracy of assumptions made during the development of crash prediction models using HSM procedures. Reliability, accuracy, and appropriateness of safety models may not always conform to necessary requirements relative to facility type. Practitioners also appear to be struggling to fully understand and appropriately communicate the benefits of the HSM and results derived from the included methods. There are many factors that affect reliability, for example model accuracy. Case studies presented at various conferences, including the Transportation Research Board (TRB) Annual Meeting, and through other initiatives demonstrate that some practitioners are utilizing the models incorrectly or in ways not recommended, and are also displaying crash prediction results without properly understanding the model reliability. Some crash prediction models have been reported with measures of model reliability while others have not. Understanding and communicating consistently reliable crash prediction results is critical to credible analysis and a barrier for some transportation agencies or professionals utilizing these models.


The objectives of this research are to develop guidance for: (1) the quantification of the reliability of crash prediction models including crash modification factors and/or functions (CMFs) and safety performance functions (SPFs) for practitioner use; (2) user interpretation of model reliability; and (3) the application of crash prediction models accounting for, but not limited to assumptions, data ranges, and intended and unintended uses. The guidance should address the following, at a minimum:
  • Methods to improve the reliability of crash prediction models
  • Implications of assumptions
  • Crash Prediction Model validation
  • Data quality
  • Use and reliability of calibrations 
  • Combining CMFs 
  • Enhanced accuracy and reliability as a result of increased model complexity 
  • Implications of crash prediction model limitations on safety programs and policy 
  • Effective communication of crash prediction model outputs to a variety of audiences
The guidance should include a number of case studies or illustrative examples that demonstrate the quantification and user interpretation of crash prediction models reliability. Examples may illustrate the application of crash prediction models accounting for, but not limited to assumptions, data ranges, and intended and unintended uses. The guidance is intended to assist practitioners and researchers in addressing the application and understanding and communicating the model outcomes. The research results may be incorporated in a future edition of the AASHTO HSM.


The NCHRP is seeking the insights of proposers on how best to achieve the research objectives. Proposers are expected to describe research plans that can realistically be accomplished within the constraints of available funds and contract time. Proposals must represent the proposers’ current thinking described in sufficient detail to demonstrate their understanding of the issues and the soundness of their approach in meeting the research objectives. The work proposed must be divided tasks and/or phases. Proposers must describe the work proposed in each phase and task in detail. The research plan should build in appropriate checkpoints with the NCHRP project panel including, at a minimum (1) a kick-off teleconference meeting to be held within 1 month of the contract’s execution date; (2) one face-to-face interim report review meeting at the end of Phase 1; and (3) web-enabled teleconferences for review and approval of other interim deliverables deemed appropriate by the NCHRP. The work proposed must be divided into at least two phases and each phase must be divided in to tasks. Proposers must describe the work proposed in each phase and task in detail, and identify specific deliverables for submission to NCHRP for review and approval.


Phase I should include: (1) conduct of a comprehensive literature review; (2) an analysis of available resource data on CMFs and SPFs; (3) preparation of a detailed work plan to identify and address relevant research gaps and develop the guidance; (4) development of an outline or framework of the guidance; and (5) preparation of an initially proposed dissemination/communication plan for the final guidance. The results should be incorporated into an interim report within 13 months of contract execution. An in-person meeting will be held with the project panel to discuss the interim report. NCHRP review and approval of the interim report is required before proceeding to any subsequent phase.


Subsequent phases will involve implementing the approved detailed work plan prepared in Phase I and preparing the final deliverables, including at a minimum (1) a final report that (a) documents the entire project, including the guidance and all other specified deliverable products of the research; (b) an executive summary that outlines the research results; and (c) the research team’s recommendation of needs and priorities for additional research; (2) a PowerPoint presentation with speaker notes describing the project background, objective, research method, findings, and conclusions suitable for use in a webinar; (3) a communications /dissemination plan to convey the findings of this project to the practitioner and research community; and (4) a stand-alone technical memorandum titled “Implementation of Research Findings and Products” (see Special Note J for additional information).

 Research in progress; phase 2 underway

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