Reading about how Roman roads were built, it seems they built drainage ditches on both sides. I've found various sites talking about this, such as this one.
This leads to a question that will sound trivial at first. Where did these ditches lead?
Did they stay parallel to the road forever? In that case I don't see how they would help with drainage at all. They need to lead to a large body of water.
So for example, as a road approached a town, did the ditches also follow the road right into town? That seems impossible, but if they terminated at the town border, then where does that lead except nowhere?
Or, as a road moved away from a town, did the ditches follow it even uphill? If so then we have drainage flowing back towards town and then terminating right at the town. Sounds like a recipe for flooding the town.
So the obvious guess is that other canals were dug to intersect the ditches and then lead to a lake/river/sea. But I have never been able to find mention of such canals. And I've never found a map of roads that include the ditches in the first place.
(I know the Romans built some canals. What I'm saying here is I can't find any mention of canals connecting to drainage ditches of a road.)
Maybe I have some fundamental misunderstanding about how drainage ditches work? I thought they must lead to a big body of water. But maybe they are more like shallow wells that allow the water to seep down into the ground (down to the tablewater) more quickly? That seems hard to believe though. That sounds like it would quickly fill up during rain and then be useless.
So just to emphasize where I'm coming from... The whole idea of roads and waterworks having to work together is confusing or at least disheartening. It would seem to severely limit where and what path you can built a road. That's why I wonder if these drainage ditches worked in some other way.
