Calibration of SPFs in the HSM, IHSDM, and SafetyAnalyst

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Calibration of SPFs in the HSM, IHSDM, and
SafetyAnalyst

• Doug Harwood
• Midwest Research Institute

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Purpose of Calibration

• To enable SPFs or safety prediction methods
  developed with data from one jurisdiction to be
  applied in another jurisdiction

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What Differences Between Jurisdictions Does
Calibration Account For?

• Climate
• Driver behavior
• Animal populations
• Crash reporting thresholds
• Crash reporting system procedures

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How is the Calibration Factor Used?

• Typical SPF with Calibration Factor
• N exp(a b x ln( AADT)) C

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How is the Value of the Calibration Factor
Determined?

• Steps in Calibration
• Select facility types and SPFs
• Select calibration sites
• Obtain data
• site characteristics
• observed crash data
• Apply SPF or predictive method to each site
• Compute calibration factor

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How is the Value of the Calibration Factor
Determined?

• C Sum of observed crashes
• Sum of predicted crashes

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Calibration in SafetyAnalyst

• The SPF for each individual facility type (i.e.,
  site subtype) is calibrated separately for each
  calendar year
• All available sites for each site subtype of
  interest are used
• Calibration is done automatically by the software
  whenever new data are loaded no user
  intervention is required

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How is the Calibration Factor Used?

• SafetyAnalyst SPF with calibration factor
• N exp(a b x ln( AADT)) C

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Data Needs for Calibration of SafetyAnalyst

• Site characteristics
• Crash frequencies
• All needed data are mandatory variables in the
  SafetyAnalyst data set

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Calibration in HSM Part C

• The entire predictive method for a given facility
  type is calibrated, rather than individual SPFs

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Calibration in HSM Part C

• Calibration procedures are presented in the
  Appendix to Part C
• Guidance is provided on
• data elements needed for calibration (listed in
  Exhibit A-2 in Appendix to HSM Part C)
• All input variables to HSM Part C methods are
  either required or desirable data
• minimum samples sizes

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Calibration in HSM Part C

• A calibration data set must be assembled for each
  facility type
• Minimum sample size for calibration
• 30 to 50 sites that collectively experience at
  least 100 crashes per year
• The same calibration sites can be used with new
  crash data (and updated traffic volumes) to
  calibrate for future years

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Calibration in IHSDM

• There is no formal calibration method built into
  the software
• Calibration factors determined outside the
  software may be entered by the user
• Calibration factors developed for the HSM or
  SafetyAnalyst can be used in IHSDM

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Questions?