Calibration of SPFs in
the HSM, IHSDM, and
SafetyAnalyst
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 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
N predicted  N spf
x
(AMF1x  AMF2x ... AMFyx ) C x
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?