This paper provides a method to classify TL3regions across OECD countries based on their level of access to metropolitan areas. TL3 regions are classified as ‘metropolitan’ if more than half of their population lives in one or more functional urban area (FUA) of at least 250 thousand inhabitants and as ‘non-metropolitan’ otherwise. The method sub-classifies metropolitan regions into ‘large metro’ or ‘metro’ regions based on the population size of the FUAs located within those regions. Non-metropolitan TL3 regions are sub-classified into: with accessto a metro, with access to a small/medium city, or remote based on their level of access to a FUA with population above a predetermined threshold. The method relies on publicly available grid-level population data and localised information on driving conditions.