6.1.3.5 PSI Analysis
PSI analysis is conducted using the HSM predictive method. The purpose of PSI analysis is to evaluate the existing conditions of the area of interest to determine the potential to reduce crashes at that location. PSI analysis uses the observed crash data at a site to apply the EB method and determine the expected average crash frequency. The difference between the expected average crash frequency and the predicted average crash frequency is the potential reduction in crashes at that site when compared to similar facility types. If the PSI is a positive value (expected crashes are more than predicted crashes) then the site experiences more crashes than would be expected and has the potential to improve safety at that location. provides a visual representation of the relationship between observed, predicted, and expected crashes when performing PSI analysis.
The following are the basic analysis steps for performing PSI analysis:
- Determine study area;
- Determine crash analysis years;
- Obtain historical crash data for study area and analysis years;
- Calculate predicted average crash frequency using the HSM predictive method for the study area site(s);
- Calculate expected average crash frequency by applying the EB method within the HSM predictive method for the study area site(s);
- Determine the difference between the expected and predicted crash frequency; and
- Document analysis methodology, assumptions, and analysis results
contains an
example
of one approach for graphically documenting the PSI analysis results.