Numetric supports each of these methods for calculating LOSS.
Level of Service of Safety (LOSS)
LOSS measures how a roadway segment or intersection performs relative to predicted crash frequency or severity using a Safety Performance Function (SPF). There are multiple ways to calculate LOSS categories, as outlined below.
LOSS using expected crash values and inverse gamma distribution
Expected crash values are compared to predicted values to produce a performance rating from LOSS I (low potential for crash reduction) to LOSS IV (high potential). Correctable crash patterns may still exist in lower categories. The percentiles are set using the inverse gamma function. This is the revised method included in the AASHTO Highway Safety Manual, Second Edition.
Source: This method is recommended in the second edition of the Highway Safety Manual.
LOSS using observed crash values and standard deviation
Observed crash values are compared to predicted values to produce a performance rating from LOSS I (low potential for crash reduction) to LOSS IV (high potential). Correctable crash patterns may still exist in lower categories. This is the version included in the AASHTO Highway Safety Manual, First Edition.
Source: This method is recommended in the first edition of the Highway Safety Manual.
LOSS using expected crash values and standard deviation
Expected (Empirical Bayes-adjusted) crash values are compared to predicted values to produce a performance rating from LOSS I (low potential for crash reduction) to LOSS IV (high potential). Correctable crash patterns may still exist in lower categories.
Source: This method is recommended in the Safety Analyst User’s Guide.