Skimming through this Im not sure that this answers this question (maybe Im missing it). Think about it from the perspective of the pollution on a square block. Does a speeding higher emitting car that leaves the block quickly leave more pollution than a slower lower emitting car that takes longer to leave the block? Im pretty sure there has got to be something to this because at the very extreme Id imagine if I idled at 1 mph down a block Id leave a lot more pollution