This article breaks down the naming convention the Idaho Transportation Department used for Route ID fields in Numetric.
Route Code
Five unique digits (based on the last five numbers of the original six-digit segment code for routes for the state roads)
Traffic Flow Direction
A (Ascending)
D (Descending)
Route Type
IN (Interstate)
US (United States Highway)
SH (State Highway)
OH (Off Highway)
PO (Polygon/Political Boundary)
Highway Number
For example, 184, 093, etc., or 000 for all other routes.