The western edge of the M30 motorway in Madrid has been buried; but the eastern part remains on surface, and the way in which it has been contorted to integrate the new tunnels and engineering variances make for a complex geometry in some areas. And probably a missed opportunity in the greening of these “wasted” fringe spaces… (each one can have his say on what is wasted here)
I think people too often pave over possible green space because they think it is too much trouble or too expensive to maintain.
As a matter of fact, under a part of this section there is a tunnel. But there is no need for full-fledged oaks. Some 100 m north there are xeroscaping elements which work just fine with Madrid climate, but this has not been used in areas which, sure, are most dificult to maintain, but have received no alternative treatment. Just some stones well placed would be fine, as a matter of fact, at least from an aesthetic viewpoint