1. SPF Development and Data Needs John Milton Ph.D., P.E., Washington State Department of Transportation July 29th, 2009 Chicago, Illinois National Safety Performance Function Summit

2. Overview • Why SPFs? • Model Development (Frequency vs. Severity) • Data Collection • Model Specification Issues • Homoskedasticity • Regression to the Mean • Omitted Variable Bias • Transferability • East Versus West

3. Why SPFs? • Early 1990s recognition that Federal Dollars were not going to last, and efficiency of expenditures had to increase • Wrote in to law that state had to address both historic and risk of a crash • Doing it already, just not using what might be considered “available science”

4. Frequency vs Severity • Frequency based • Poisson/Negative Binomial • Bayes (Hierarchical) • Nested and Mixed Logit

5. Key Data Components Necessary

6. Data in Washington • Geometric • Horizontal/Vertical Curve/angle point radius, length, PC/PVI/PT • Lane and Shoulder width • Pavement type and condition • Accident data • by severity, type, weather, contributing factors, actions etc. • Traffic • ADT, PHF, Truck %, etc

7. Development of SPFs in WA State • Homogenous vs non-homogeneous sections • New section based on changes in section • Categorical versus Continuous variable • E.g., Shoulders width greater than 5’ versus actual shoulders width • The greater the use of continuous the more data needed • Chose homogeneous sections with a preference towards continuous variables

8. Development of SPFs in WA StateStatistical Issues • Chose Homogenous sections to reduce heteroskedasticity (unequal variance) in models • Could use continuous data more readily • Prefer well specified, local models to ADT only models because of omitted variable bias in models, and low goodness of fit.

9. Development of SPFs in WA StateStatistical Issues • Concern about transferability across state • Functional class • East/west • Intersection data greatest challenge because of minor street ADT

10. Development of SPFs in WA StateStatistical Issues • Severity Models • Roadside information necessary • By Severity Type • Some severity levels may be grouped

11. Lessons from Past Experience 1 2 3 4 5

12. Summary Develop Data collection plan consistent with states capability and desires It is ok to start slow and add as you go along It is not necessary to develop your own SPFs. There are benefits and disadvantages to doing this in terms of cost, data resources and upkeep Training is necessary

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