Why it looks like this

Venice is the puzzle where the roads layer fails you on purpose. The city stands on 118 low islands in a lagoon, connected by around four hundred footbridges — and by canals that do everything streets do in other cities. The only road is the causeway from the mainland, which is why the map shows one thin line feeding an almost empty square.

Turn on the water layer and the city appears: the Grand Canal's reversed S — the main road of Venice for a thousand years.