if you are driving with no other cars on the road in front of you, it is more severe that highway of course, but its when you are driving through dust that is bad bad, or if your particluar vehicle kicks alot of dust into the engine bay...
I have to drive half mile to the highway from my place, honda crv, i dont drive fast, my air filter is no dirtier that driving anywhere else. Running an amsoil EA air filter myself.