It completely can eat up all that mileage. I mean look at all the new cars even with their '40 mpg highway' ratings, but barely eeking out mid to low 20s for city mileage. That is the penalty for idling and using your brakes so much. Commuting in a city takes rediculously small amounts of power when you look at it vs highway driving. This is one reason hybrids and electric vehicles get so much better city mileage than normal cars, because they can recoupe that braking energy, and they don't idle while waiting for a light. It really does eat up almost half your tank of gas.