Calibration of SPFs in the HSM, IHSDM, and

SafetyAnalyst

Doug HarwoodMidwest Research Institute

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

Questions?