say you are in a car travelling at 60 mph, and another car beside you is travelling at 80 mph about to overtake..If a third really fast car was to zoom past at 200 mph (in a third lane), to me he is effectively travelling at 140 mph and to the overtaker 120 mph.. But the speed of light would be the same to all of us, if a car travelling at the speed of light was to whizz past also...why? why is it an exception? and exactly at what speed threshold can we stop subtracting speeds like I did above? c-2 kph? c-3 kph?