Safety Performance Function
Guidance
Raghavan Srinivasan & Daniel Carter
UNC Highway Safety Research
Center
Karin Bauer
MRIGlobal
Objectives
• Develop and provide information to assist
state and local agencies calibrate the
Safety Performance Functions (SPFs) in
the Highway Safety Manual (HSM) and/or
develop their own SPFs for their state or
region
Tasks
• Task 1: Develop work plan and conduct kickoff
meeting
• Task 2: How to Choose Between Calibrating
SPFs from the HSM and Developing
Jurisdiction-Specific SPFs (SPF Decision Guide)
• Task 3: ‘How-To’ Guidebook for States
Developing Jurisdiction-Specific SPFs (SPF
Development Guide)
Other Related Efforts
• NCHRP 20-7 (Task 332): User’s Guide to
Develop HSM SPF Calibration Factors
(Dr. Geni Bahar) (SPF Calibration Guide)
• SPF Needs Assessment (Volpe)
– Interviews with selected States to assess
needs regarding SPFs
• Safety Performance Functions: A
Workshop (Dr. Ezra Hauer)
– Fundamentals of about prediction models
– Use of Excel
SPF Decision Guide
(FHWA)
Calibrate
SPF Calibration Guide
(NCHRP 20-07)
Calibrate or
Develop SPFs?
Develop
SPF Development
Guide (FHWA)
SPF Workshop
(Ezra Hauer)
SPF Decision Guide
• Background and Context
– Intent
– Related documents
• What are SPFs?
– Definition
– Example
• How are SPFs used?
– Project level analysis
– Network screening
– Evaluating the effects of engineering treatments
SPF Decision Guide, contd.
• Options for Obtaining SPFs
– Calibrating existing SPFs
• SPF sources
• Calibration process
• Steps in calibration
– Developing jurisdiction specific SPFs
• Steps in developing SPFs
SPF Decision Guide, contd.
• Decision process: how an agency obtains
SPFs
– Step 1: Determine intended use of SPFs
– Step 2: Determine facility type
– Step 3: Identify existing SPF
– Step 4: Consider sample size necessary for
calibrating SPF
– Step 5: Consider roadway data necessary for
calibration SPF
SPF Decision Guide, contd.
– Step 6: Calibrate existing SPF
– Step 7: Assess quality of calibration factor
– Step 8: Consider statistical expertise for
developing SPF
– Step 9: Consider sample size necessary for
developing SPF
– Step 10: Determine crash type to be
addressed by SPF
– Step 11: Develop SPF
SPF Decision Guide, contd.
• Level of effort estimates and sample size
– Sample size and level of effort estimates have been
provided
• Project level
– Calibration
– SPF development
• Network screening
– Calibration
– SPF development
– Estimates based on judgment of the project team
SPF Development Guide
• Background and Context
– Intent
– Related documents
• What are SPFs?
• How are SPFs used?
– Project level analysis
– Network screening
– Evaluating the effects of engineering treatments
• Before-after
• Cross-sectional models
SPF Development Guide,
contd.
Statistical Issues
• Overdispersion
– Poisson versus negative binomial
– Form of the overdispersion parameter
• Selection of explanatory variables
– Purpose of the SPF
– Missing variables
• Functional form
– Interaction terms
– Possible approaches
– Cumulative residual plots
SPF Development Guide,
contd.
Statistical issues, contd.
• Overfitting of SPFs
– Cross-validation
– Relative goodness of fit measures (AIC, BIC)
• Correlation among explanatory variables
• Homogenous segments and aggregation
• Presence of outliers
– Cook’s D
– CURE plots
SPF Development Guide,
contd.
Statistical issues, contd.
• Endogenous explanatory variables
– Explanatory variables not really “independent”
• SPFs for different crash types and severities
–
–
–
–
Proportion of crashes by type
Separate SPFs
Multivariate analysis
Model proportion by severity
• Goodness of fit
SPF Development Guide,
contd.
Steps Involved in Developing SPFs
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•
•
•
•
•
•
•
Step 1: Determine use of SPF
Step 2: Identify facility type
Step 3: Compile necessary data
Step 4: Prepare and cleanup database
Step 5: Develop the SPF
Step 6: Develop the SPF for the base condition
Step 7: Develop CMFs for specific treatments
Step 8: Document the SPFs
SPF Development Guide,
contd.
Recent advances in SPF development
and estimation
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•
•
•
•
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Variance of crash estimates obtained from SPFs
Temporal and spatial correlation
Model forms
Generalized additive models
Random parameters models
Bayesian estimation methods
Software tools for estimating SPFs