Method and system for zone based detection of vehicles passing into and out from a geographical zone using polygons to define the border of said zone, wherein a first polygon (32) defines a virtual border against which vehicles passing into the zone is compared by use of a GNSS system (10) comprising an OBU in every vehicle to be surveyed by the method. A second polygon (31) larger than the first polygon (32) and completely enveloping said first polygon is defined as a virtual border against which objects passing out from the zone is compared by use of the GNSS system.