First you have to have a some ducts, like a bunch of tear ducts. You can usually catch this kind of ducts while they are crying and can't see very good. Then you make them into a road using duck tape to stick them together. Usually you need more road than you have ducts, so a traditional Roman-style viaduct uses holes also. This is the Holey Roman style. The holes are arranged to make arches, and the road goes over the top and is supported by the holes. You can also have a viaduct for trains.