Calibration of SPFs in the HSM IHSDM and

Calibration of SPFs in the HSM, IHSDM, and Safety. Analyst Doug Harwood Midwest 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 SPFs 2. Select calibration sites 3. 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 Safety. Analyst • 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? Safety. Analyst SPF with calibration factor: N = exp(a + b x ln( AADT)) C

Data Needs for Calibration of Safety. Analyst • Site characteristics • Crash frequencies • All needed data are mandatory variables in the Safety. Analyst data set

Calibration in HSM Part C • The entire predictive method for a given facility type is calibrated, rather than individual SPFs

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 Safety. Analyst can be used in IHSDM

Questions?