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APPENDICES 132 Appendix B: Glossary This chapter defines the terms used in the Highway Safety Manual User Guide. Empirical Bayes Method: Method in which the evidence about the true state of the world is expressed in terms of degrees of belief (Bayesian probabilities). This method incorporates knowledge from history or other sites to obtain the best estimation. Then, the method considers the likelihood of certain types of events as part of the analysis process. Last, the method uses the Bayes theorem to convert probabilistic statements into degrees of belief instead of the traditional confidence interval interpretation. Crash: The HSM definition of a crash is a set of events that result in injury or property damage due to the collision of a motorized vehicle with another motorized vehicle, bicyclist, pedestrian, or an object. Crash Estimation: The term crash estimation is related to the methodology used to predict the crash frequency of a existing roadway for existing conditions or alternative conditions during a past or future period, or to predict the crash frequency of a new roadway for given conditions for a future period. Crash Evaluation: The term crash evaluation relates to determining the effectiveness of a particular treatment or treatment program after its implementation. Crash Frequency: The HSM definition of crash frequency is the number of crashes occurring at a particular site, facility, or network in one year. Crash frequency is calculated as the number of crashes divided by the period in years, and the unit is number of crashes per year. Crash Severity: See KABCO definition. KABCO: Crash severity scale, which provides five levels of injury severity. Even if the KABCO scale is used, the definition of an injury may vary between jurisdictions. The five KABCO crash severity levels are: Fatal injury (K): an injury that results in death; Incapacitating injury(A): any injury, other than a fatal injury, that prevents the injured person from walking, driving, or normally continuing the activities the person was capable of performing before the injury occurred; Non-incapacitating evident injury (B): any injury, other than a fatal injury or an incapacitating injury, that is evident to observers at the scene of the crash in which the injury occurred; Possible injury (C): any injury reported or claimed that is not a fatal injury, incapacitating injury, or non-incapacitating evident injury and includes claim of injuries not evident; No Injury/Property Damage Only (O; also known as PDO). Effectiveness: The term effectiveness refers to a change in the predicted average crash frequency or severity for a site or project. Expected Average Crash Frequency: This term is used to describe the average crash frequency, under a given set of geometric design and traffic volumes for a given time period, of a site or network. Observed Average Crash Frequency: This is the historical average crash frequency at a given site. Predicted Average Crash Frequency: This is the average crash frequency at a site or network obtained with the application of an SPF for the study period under the given conditions.
APPENDICES 133 Predictive Method: This term refers to the HSM Part C methodology used to estimate the long-term average crash frequency of a site or network under given geometric design and traffic volumes for a specific number of years. The result from the predictive method is the expected crash frequency.
