Originally Posted by RossFW
The treadmill has no ground equivalent
. This means that the belt is not the road, but the Earth. This is because the windspeed status must be forced upon the observer. That means the belt is also a privileged and unique perspective. If not then the treadmill collapses to the real world equivalent of a real treadmill, with different objects at different velocities in the same frame. Either way, no motion.