He could have gotten off at the exit. Right when the clip starts it shows an on ramp. The rig could have lost it right before that and exited. That sight would have prompted the person to break out their camera.
Edit: you can see it passing the white lines on the road. It doesn't appear to be going very fast. What's the standard spacing on those lines. Shouldn't be hard to make a good guess on its speed with a simple calculation.

