I agree with you about roadside ditches and drains.
I live in an area with chalk sub soil, so we rarely have more than temporary, localised flooding. The floods we do have could be prevented by clearing the roadside ditches, which I believe are the responsibility of the farmers, and replacement of drains, some of which are Victorian.
I was told that trees are needed on the hillsides (not round here), because the roots would help soak up the water, so less would run down into the valleys or, at least, slow it down. Apparently it's a problem in the North West (Lake District, etc).