CG I think would have a lot to do with this technique. You could also shoot the bus going in front with the man far away and then shoot the bus again a second time this time with the man closer. Then piece the two different shots together at the point where the bus passes in front. Thus giving the illusion that the man came closer rather quickly.

(a) Camera on the tripod so it doesnt move. Bus goes by twice. Once for each position. Stitch the shots together.
(b) Film character without bus. Have character move. Film them again in the new position. Add a bus in post to cover the transition. Since nothing on the bus really moves and it'll be going quickly you could just use a photo. Might be better than waiting for the next bus to come by.

RJSchwarz

first, you film a clean plate of the scene, or a scene with nothing but the background

next, you film youre actor in the first position (in this case, farther from the car)

next, film youre actor in the second position (closer)

now, film the vehicles moving across the screen.

be careful not to move the camera or tripod AT ALL!

okay, now that you got that done, take the footage into which ever effects processing software you have, and mask the clips together, making sure to put the actor footage in the appropriate positions of the timeline.

Honestly I think lots of the time it's just a clever cut. Go out and take some test footage. I think RJSchwarz has got it down. I've done similar cuts even with different cars; it was just the continuous motion that cemented the cut. If you can think of a specific example with the cut, go back and analyze it. Chances are it's just a clever cut, probably (like rjs said) a cut between two similar bus shots.

http://vimeo.com/corax

Actually this can be done even simpler than over lays and double shooting scenes to put it all together in the end.

Only two shots are needed.

Shot #1) The actor walking in the distance, then the bus passes by.

Shot #2) A much closer different angle on the actor as Actor approaches corner.

Editing:
Shot #1 plays out, until the last frame of the bus covers the screen. This is where the edit is done. THe bus must fill the the whole frame, and then edit in. Shot number #1. The closer shot of the ACtor -Viola! you have your scene. Our minds let the bus pass, an the edit works well. It' a bit tricky to get the timing just right on the edit, but a few tries, and it will look good.

The other methods would work as well, but this is an easy easy way to do it.