Traffic Boroughs – or simply boroughs – are largely a London-based concept where traditionally the administrative boroughs within and around London may have been split into one or two traffic boroughs (eg Wandsworth within Central London, separated from the rest of Wandsworth), with further definitions for areas outside London being aggregates of (non-London) boroughs (e.g., the whole of the North of England and Scotland might constitute a single traffic borough).
Within the current TfL (Transport for London) SATURN models, there are currently 45 boroughs numbered sequentially from 1 to 45, with numbers 1 to 33 representing the 33 administrative sub-areas of Greater London on a one-to-one basis (no splits), 34 to 40 adjacent or near counties, and the rest large regions further afield.
N.B. Like groups there should be no borough 0.