how did we do it? we did it for real and built a road in glass. with almost every shot achieved in-camera using only compositing to join the background and foreground plates. we devised a stereoscopic technique using two cameras to provide information in the clean-up to remove the grid that was holding the massive glass sheets 3m above the ground.
the tricky part was choosing the right lens and distance from the glass and working out a choreography that was interesting from beneath. too close and it was no longer spectacular, too wide and the choreography of the performers became too much like ants so you didn’t care. a CG previz really helped us to make these decisions.