Booleans Mini Tutor: Impression

[maker]

Presumably I'm missing something here ...
pp

You aren't. I am - in my top storey You got to the bub of the problem. I should have realised that, while it 'looks' okay, the contour levels at the two sides of the road are not necessarily the same.

How to flatten it for corresponding points on the curve? Extrude one side-face? Or make the impression from one side only, and work from that? Or some variation of ggaliens' suggestion?

Or leave it to PP as an idea for another of his brilliant how-to's? Yes, I think that would be the best idea

[Actually, that's an interesting problem. It should keep me busy in my spare time. IF I do find answer I doubt it will be a mini-tutorial though.]