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 defines a virtual border against which vehicles passing into the zone is compared by use of a gnss system comprising an obu in every vehicle to be surveyed by the method . a second polygon larger than the first polygon 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 .