The California Museum of Science and Industry (now California Science Center) had an O scale diorama layout that was about three feet deep. Here's how they handled roads using forced perspective, using 1:64 cars at the rear. The road at the back of the diorama (left side of the second photo) was a chain-driven highway using Matchbox cars (1:64) moving along at a slow pace. It was a work of art, now long gone.
