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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. Overview. Why SPFs? Model Development (Frequency vs. Severity) Data Collection

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spf development and data needs

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

overview
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
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 vs Severity
  • Frequency based
  • Poisson/Negative Binomial
  • Bayes (Hierarchical)
  • Nested and Mixed Logit
data in washington
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
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
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.
development of spfs in wa state statistical issues1
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
development of spfs in wa state statistical issues2
Development of SPFs in WA StateStatistical Issues
  • Severity Models
    • Roadside information necessary
    • By Severity Type
      • Some severity levels may be grouped
summary
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