The greater part of these costs stem from road transport. Costs per traffic unit are highly variable, depending in particular on the extent of urbanisation concerned. The following ranges and orders of magnitude can be proposed: one passenger-km has a social cost about the same as one tonne-km; the cost by road (car or truck) is about 0.02 ECU per unit-km; the cost by rail is about 10 times less, and of the same order of magnitude as the cost by air. These values are too imprecise for it to be possible to distinguish any time trends or differences between countries.