Calibration of SPFs in the HSM, IHSDM, and SafetyAnalyst
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Transcript of Calibration of SPFs in the HSM, IHSDM, and SafetyAnalyst
Purpose of Calibration
• To enable SPFs or safety prediction methods developed with data from one jurisdiction to be applied in another jurisdiction
What Differences Between Jurisdictions Does Calibration
Account For?
• Climate• Driver behavior• Animal populations• Crash reporting thresholds• Crash reporting system procedures
How is the Calibration Factor Used?
Typical SPF with Calibration Factor:
N = exp(a + b x ln( AADT)) C
How is the Value of the Calibration Factor
Determined?
Steps in Calibration:1. Select facility types and SPFs2. Select calibration sites3. Obtain data:• site characteristics• observed crash data
4. Apply SPF or predictive method to each site
5. Compute calibration factor
How is the Value of the Calibration Factor
Determined?
C = Sum of observed crashes Sum of predicted crashes
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
How is the Calibration Factor Used?
SafetyAnalyst SPF with calibration factor:
N = exp(a + b x ln( AADT)) C
Data Needs for Calibration of SafetyAnalyst
• Site characteristics• Crash frequencies• All needed data are mandatory
variables in the SafetyAnalyst data set
Calibration in HSM Part C
• The entire predictive method for a given facility type is calibrated, rather than individual SPFs
xyx2x1xx spfpredicted C)AMF...AMF(AMFN N
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
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
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