an area of the customs territory adjacent to the land frontier, the extent of which is determined in national legislation and whose limits serve to distinguish frontier traffic from other traffic, a zone which, as the crow flies, does not extend more than 15 kilometres from the frontier. The local administrative districts, part of whose territory lies within the zone, shall also be considered to be part of this frontier zone, a zone extending to a depth of 25 km as the crow flies from the frontier common to two Member States