If he's going faster than the truck, then wouldn't he then be going "forward" at that point? He would no longer be moving backward relative to the ground, but forward at his speed minus the truck's speed. I think...

I work at Starbucks, and no they don't tell Starbucks employees to correct people; in fact they actively discourage it. No one I've ever worked with cares if people ask for a small/medium/large; I always did before I worked there. If you are ever asked if you meant grande or venti if you ordered a large, it is because sometimes customers say a large but really want a 'grande', and mention it when you give them a Venti. I assume they are just used to the medium/large/extra large menu items from other places. What I do ask about is what people mean when they order a regular coffee, as everyone seems to have a different opinion on whether it is a small or a medium.