> I imagine some of the Roman stuff was built on even older roads and channels.

Watling street was paved by Romans, but at least part of the route was a road used by local Britons long before the Romans invaded.

Lidar has been a game changer. Also, in the UK context, new developments will often require archeologists to survey the land before ground works can start. There are piles of photographs of fields from the RAF, US air force and Luftwaffe. And names of places and roads often reveal ancient origins. If you meet somewhere with chester or caster in it, e.g. Manchester, that suggests a Roman origin. The same for stret and strat.