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

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

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”

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

Key Data Components Necessary In-house expertise University of Washington Staff Most data readily available Potential additional sources Srview 3 SRview360 Ongoing statewide roadside data collection Data needs : primarily intersection ADT for minor roads (will require use of non-existent funds) Data

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

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

Development of SPFs in WA State Statistical 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.

Development of SPFs in WA State Statistical Issues Concern about transferability across state – Functional class – East/west Intersection data greatest challenge because of minor street ADT

Development of SPFs in WA State Statistical Issues Severity Models – Roadside information necessary – By Severity Type Some severity levels may be grouped

Lessons from Past Experience The more complex the model is to the user the more challenges will occur. Models will be evaluated and question for deviations from current observations Models benefit from good data and concerns for specification errors, not just RTM! Self developed models can be under or over specifiedTraining is a necessity

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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