Method for updating a digital street map

The method involves equipping two vehicles moving about a road system (1) with respective onboard unit generating position readings. Average path length is calculated for the road segment based on first and second path lengths. The average path length is stored as updated stored segment length of the road segment in a digital road map (1') if the calculated average path length deviates from the stored segment length of the road segment by a predetermined minimum amount and/or by no more than a predetermined maximum amount. Independent claims are also included for the following: (1) an onboard unit included in a vehicle moving about a road system (2) an apparatus for updating a digital road map representing a road system and in which road segments having segment lengths are stored.