[Stiglr]

Hard to really see what you've got there, but....

Try this perhaps. Not *sure* it'll work, but maybe...

Enter vertice mode and select all the vertices on the bottom edges of the walls (thus outlining the road surfaces). Then select "Create convex hull".

You may get a lot of extra surfaces you don't want, but you should be able to select and delete large groups of them until you're left with your track surface. Or, if that's way too messy, try doing it in smaller chunks a section at a time, and each time you do a new length of road, Merge it with the road surfaces you created earlier.