SPFs Applications by State DOTs

John Milton Ph.D., P.E.,Washington State Department of Transportation

National Safety Performance Function SummitJuly 29th, 2009Chicago, Illinois

Overview

• Washington State Applications– Rural Two Lane Highways– Interstates– Signals and Channelization

• Ongoing Development– Rural Multilane Highways

• Present Uses

Effective Expenditure of Dollars

• Bottom line is to maximize potential for return on investment

• Approaches range from standards based solutions to focused solutions

• Return on investments decisions are critical• 90% Federal Investment no longer• Suggest the need for optimized decision

making

Effective Expenditure of Dollars

• In standards based solutions one requires a long return.

• Focused solutions require a strong ability to determine the expected safety picture.

• Anecdotal is not acceptable, nor is a low probability of a return– Rate based or methods that don’t control for

specification error will not optimize return

Development of SPFs in WA State

• WSDOT chose to move toward development of local SPFs– Developed for all highways excluding interstate– Believe that statistical issues related to rate based

or short term frequency estimations need to be considered in program development and these elements phased out over time

– Prefer well specified, local models to ADT only models

Development of SPFs in WA State

• Use SPFs for planning and programming in the prevention sub-program for both corridors and intersections

• Models were developed independently for each element

• Modifications were made in Design Manual to account for changes, and the particular focus of the program

Development of SPFs in WA State

• Over time WSDOT moved from a focus on all severity collision to fatal and serious

• With interstate development use Data Envelopment Analysis to allow for modifications of policies within program needs

Development of SPFs in WA State

• Rural two lane highways early development in 1994

• Previously used critical rate solely– Used negative binomial estimation– Homogenous sections– Entire rural highway systems for collectors, minor

arterials and principle arterials with East/West split.

Development of SPFs in WA State

• Rural two lane highways– Large Data Set

• Geometric• Traffic• Crash

– Analyst wanted to use to determine before and after even when told not to do so.

Development of SPFs in WA State

• Interstate highways– Large Data Set

• Geometric• Traffic• Crash• Weather

– Hierarchical Bayes– Used to analyze entire network– Data Envelopment Analysis to allow for flexibility

in policy

Development of SPFs in WA State

• Rural Multilane• Refinement of two-lane models

Development of Severity SPFs in WA State

• Multilane divided highways• Using multinomial, nested and mixed logit

estimation• Mixed Logit offered flexibility• Allows for estimation of coefficients and

variance• Developed utility functions (SPFs) for PDO,

Minor and Major Injury• Future to incorporate full roadside database

Future Relationships to HSM, SafetyAnalyst and IHSDM

• WSDOT intends to adopt the tools consistent with current WSDOT Policy that prefers SPFs

• WSDOT will encourage – use in Developer Review, Local Agency Evaluations

and EA/EIS Statements– continued growth in methods and procedures,

with flexibility to use local SPFs as an important element

• Severity Models using Roadside Features will occur

Lessons from Past Experience

•SPFs can improve efficiency of expenditures

•SPFs are currently available with HSM, SafetyAnalyst, and IHSDM and useable

•SPFs can be developed relatively easily if data is available

•SPFs have multiple uses in the project development context

•Training is a necessity

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Summary

• It is ok to start slow and add as you go along• Depending on the use of the tools, data

collection may not be as expansive as once thought

• Training is necessary• Think outside the box and be willing to move

towards safety as more than an anecdotal consideration

THE END