Yep, I agree, problem then becomes one of, Just how big am I willing to make a map? A running fight down the road is easy to improvise. I use Vinyl mats, so it is simple to just draw the road and say "There is thick forest on either side" and go from there.

Also, while yes the PCs could see the ruined wagon from a bit up the road, the main encounter will start at the wagon, if not I have other maps to lay down at other area.