Hmm...never thought about it like that before. You may have a point. However, I think that's a bit flawed. In your car example, you COULD say that the Earth was accelerating and you were standing still. But, you can rule that out with simple deductive reasoning - your foot was pressing on the gas peddle in the way that always makes you go 50 MPH, not to mention that all the kids in that school zone would notice if they suddenly started accelerating 50 MPH faster. Therefore, while it's possible by some incredible stretch of imaginaton that everything else was going 50 MPH, it's ridiculously unlikely.
That ties in, again, with the Cavendish experiment. If you were on the masses that gravitate/accelerate towards each other, you could say that the masses were accelerating OR you could say the Earth was accelerating. However, again, that's amazingly unlikely if you just think about it. The Earth has never been known to change acceleration (well, never been known to change speed, it's always undergoing angular acceleration), and though the acceleration would be much less noticable for people on Earth then in your school zone example, there would still be effects. Also, the acceleration would be going in two different directions, since the two masses are accelerating in different directions, and who knows what that would do to the Earth? Probably not rip in two, but if one half of Earth started going one direction and the other half started going the other...
Basically, while there's a tiny, tiny chance that everything else is moving, it's almost absurd to claim so, since there's very good reason to believe that you were the one accelerating. It's like thinking a magical pixie that lives in a mirror paints your picture on the mirror from the inside several dozens of times a second instead of the light just reflecting off the mirror back to your eye; possible, but not at all probable.