1. A method for determining the length of a path traveled by a vehicle using an onboard unit carried by the vehicle, the unit generating a sequence of readings of an own position thereof and a related position quality index for each position reading of the sequence, comprising: extracting a set of consecutive position readings from the sequence, disregarding those position readings, the position quality indices of which fall below a predetermined first threshold value; determining a set quality index based on the number of position readings of the set or based on the position quality indices thereof; and if the set quality index exceeds a predetermined second threshold value: determining the path length based on mutual distances of consecutive position readings of the set; otherwise: determining the path length using a digital road map, which is composed of road segments having respective known segment lengths, by associating the position readings of the set with at least one road segment, and determining the path length based on the segment length of the at least one road segment.