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98 Chapter 9. Summary and Conclusions The 1st edition of the Highway Safety Manual (AASHTO, 2010) is the product of over 10 years of effort and thousands of volunteer hours to provide fact-based, analytical tools, and techniques to quantify the potential safety impacts of planning, design, operations, and maintenance decisions. Part C of the 1st edition of the HSM contains the predictive methods for rural two-lane roads, rural multilane highways, and urban and suburban arterials. Since the publication of the 1st edition, two chapters with crash prediction models (CPMs) for freeways and ramps that were developed through NCHRP Project 17-45 have been approved for inclusion as a supplement to Part C. The 2nd edition of the HSM is expected to be published in the next two years. The 1st edition of the HSM does not include methods to consistently convey model reliability. During initial implementation of the 1st edition of the HSM, model results have been generated and utilized without fully understanding and communicating the accuracy of the model results, which can erode the credibility of this new and rapidly growing field. Since the publication of the 1st edition of the HSM, 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. Practitioners are also striving to fully understand and appropriately communicate the benefits of the HSM methods and results derived from these methods. Based on a survey conducted in the beginning of NCHRP Project 17-78, this study produced guidance and procedures focusing on the following model-related and application-related factors: ï· Procedures for Quantifying the Reliability of Crash Prediction Model Estimates with a Focus on Mismatch between CMFs and SPF Base Conditions ï· Procedures for Quantifying the Reliability of Crash Prediction Model Estimates with a Focus on Error in Estimated Input Values ï· Procedures for Quantifying the Reliability of Crash Prediction Model Estimates with a Focus on How the Number of Variables in CPM Affects Reliability ï· Reliability Associated with Using a CPM to Estimate Frequency of Rare Crash Types and Severities: Overview of the Problem with Possible Solutions ï· Reliability Associated with Predicting Outside the Range of Independent Variables: Problem Description and Procedure for Practitioners ï· Reliability Associating with Predictions Using CPMs Estimated for Other Facility Types: Problem Illustration with Possible Solutions Accompanying this report are the following documents: ï· NCHRP Research Report 983: Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results ï· A communications plan ï· A one-page flyer to promote the Guide Although this study addressed many of the factors related to reliability of CPMs, there are issues still to be addressed. Some of these were discussed in Chapter 1 as part of Table 1 and Table 2. Ongoing NCHRP Project 17-93, âUpdating Safety Performance Functions for Data-Driven Safety Analysis,â is addressing some of the issues related to updating and calibrating CPMs for different purposes. Issues related to CMFs including the reliability of CMFunctions and procedures for combining multiple CMFs are partially addressed in Carter et al. (forthcoming).
