WHy

is it that when im in a car traveling at some speed and i observe another car traveling at my speed plus some arbitrary amount (lets say +5mph) i observe that cars motion at less than the actual difference in speed between my car and it.

tl;dr

why do cars going alot faster than me on the highway look like they are going maybe 3-4mph past my window

>>1

OP don't listen to these assholes. It has to do with relativity. As you approach the speed of light time slows down to a stop. Obviously you're not anywhere near the speed of light but you can still observe this effect. Hope that helps.