Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
The most basic form of calibration factor for intersections is determined by following the
procedure:
• Identify a random sample of intersections that correspond to the SPF available
(ex., urban, three-leg, unsignalized intersections). Larger sample sizes will
produce more reliable calibration factors, but at a greater cost.
• Using the ADTs for the intersecting roads and the borrowed SPF, calculate the
total number of crashes (N expected ) expected at all of the intersections in the
sample.
• Calculate the sum of all crashes (N actual ) that occurred at all of the sample
intersections.
• Calculate the calibration factor (C f ) by dividing N expected by N actual .
It is important that the sample be representative of the different geometric and traffic
conditions that occur at the selected intersection type. Stratification of the sample may
assist in this regard. Harwood et al (2000) suggest that the sample sizes contain a
minimum of 100 intersections for stop-controlled intersections, and 25 intersections for
signal-controlled intersections.
Calibration using more advanced methods, or additional data is possible, but is not
explained herein. Similarly the calibration of SPFs for road segments is not explained
herein. Analysts who want to learn more about calibration and transferability of SPFs are
referred to Harwood et al (2000), and should monitor the research being conducted by the
United States Federal Highway Administration for the Interactive Highway Safety
Design Module (IHSDM).
Page 207