1. 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

2. 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

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

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

5. 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

6. 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

7. 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

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

9. 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

10. 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

11. 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

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

13. Questions?