Doug Harwood Midwest Research Institute

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Presentation transcript:

Doug Harwood Midwest Research Institute Development of State or Local Agency SPFs for Use in the HSM, IHSDM, and SafetyAnalyst Doug Harwood Midwest Research Institute

Need for State and Local Agency SPFs HSM Part C, IHSDM, and SafetyAnalyst all include SPFs that can be calibrated and used by any jurisdiction Jurisdiction-specific SPFs, if available, are desirable and may be used, but are not required

Development of State and Local Agency SPFs State and local agency SPFs must be developed properly to be valid and compatible with software tools

Available Guidance on SPF Development Section A.1.2 in the Appendix to HSM Part C SafetyAnalyst guidance document

Data Needs for SPF Development Site characteristics data to define facility types of interest Site length (for roadway segments) Traffic volumes (AADTs) Crash frequency (by severity level) Other potential predictor variables

SPF Development Guidelines Select sites that meet appropriate facility type definitions Assign crashes to roadway segments and intersections per HSM guidelines Use a valid statistical technique If the SPF will be used with AMFs, use sites with appropriate base conditions or convert completed SPF to appropriate base conditions

SPF Development Guidelines Use crash frequency, not crash rate, as the dependent variable Make sure that the SPF incorporates the effect of traffic volumes, which are typically nonlinear: AADT for roadway segments major- and minor-road AADTs for intersections

SPF Development Guidelines Use an appropriate functional form that is compatible with the software tool

Statistical Techniques Statistical techniques used for SPF development must be appropriate for the nature of crash data: Ordinary least squares regression is NOT appropriate – crash data do NOT follow a normal distribution Poisson regression is more appropriate, but the variance of crash data is not generally equal to the mean

Statistical Techniques Crash data are normally overdispered meaning that the variance of the data is larger than the mean: negative binomial regression is appropraite for modeling such data negative binomial regression provides an overdispersion parameter that is needed in software tools

SafetyAnalyst Guidelines An 8-page guideline for SPF development has been created for SafetyAnalyst this guideline is also applicable to SPF development for HSM Part C and IHSDM

SafetyAnalyst Guidelines What SPFs Are Needed? Functional Form of SPFs Data Needs for Development of SPFs Statistical Assumptions and Software References

Questions?