As a measure based on urban areas and their commuter hinterland they are a form of Metropolitan Area, though as methods of calculation differ they cannot directly be compared with other specific measurements such as Metropolitan Statistical Areas in the United States.
TTWAs have no legal status. However, they give planners and geographers an alternate view of urban life as their boundaries are tied not to arbitrary administrative limits but socio-economic ties. Having an idea of where people commute from for work is particularly useful for public transport planning.
A Travel to Work Area is a collection of wards for which "of the resident economically active population, at least 75% actually work in the area, and also, that of everyone working in the area, at least 75% actually live in the area". According to this measure, there were 308 TTWAs within the United Kingdom in 1998.
This has greatly increased the amount of information available about Travel to Work Areas, although the State of the Cities only publishes data for the 56 Travel to Work Areas based around Primary Urban Areas in England. Travel to Work Areas in Scotland and Wales and those covering only rural areas are not included.
To increase the range of statistics available the State of the Cities also publishes data for Travel to Work Areas approximated to local authority boundaries. These areas can differ considerably from the more accurate ward-based areas.