As the Highway Safety Manual (HSM) continues to evolve, determining the potential severity at a location becomes an increasingly vital component to determining safety performance. For safety performance functions (SPFs) to be reliable they must be consistent. Consistency in how states apply stand-alone severity or severity with frequency based tools and estimates are a fundamental requirement for the adoption and use of the HSM and its associated tools. Consistency is a function of several factors: (a) results that are in general agreement from a multitude of analytical techniques available to practitioner, (b) availability of data sources that allow for broad state and interstate analysis, and (c) interpretability of results for policy application at the national, state and local agency levels. Severity analysis tools as currently available in the HSM do not fully meet this definition of consistency for various reasons, but primarily originating from the fact that the various analytical techniques available in the published literature do not agree in terms of consistency in estimation of crash severity probabilities and crash severity frequencies. Three factors were critical to the adoption of safety analysis techniques used in the HSM: (1) integrity with network screening methods (Part B of the HSM), (2) data availability and requirements, and (3) model predictive performance (Part C of the HSM). The differences in severity estimations can be significant, from the methods currently in use that adopt the observed severity ratios to emerging methods that analyze the severity aspects at multiple scales from the spot, corridor, or network crash levels.
The primary objective of this research is to evaluate the various severity estimation methods in terms of the development of reliable safety performance functions. Crash prediction model results are currently being used to make project level and programmatic decisions without complete understanding of their reliability. Specific examples include lack of understanding of compounding errors due to lack of accuracy in severity estimations, use of models at or near their known limits, poor understanding of model limits, and inclusion of crash severity ratios to geographies where severity ratios as well as severity type definitions may not be transferable. Continued use of models in this way may lead to suboptimal design of projects, degradation of model credibility, and open concerns of liability and public trust. The guidelines developed with this research could both address these concerns and promote better-informed engineering judgment in the model application, greater implementation of crash prediction models, and better acceptance of model results, leading to improved decision making.
The guidelines developed with this research will be able to be applied to existing crash prediction models and will serve to improve all future models and model elements throughout the HSM and associated tools.