If you move faster than the egg hatching speed you won't get ANY progress on your eggs. By going slow in a car you can technically drive and still gain some progress on your eggs, as long as your avatar in-game is not running.
It's more complicated as to the question of incense, as there actually traversing with some speed increases your rate same as walking, but even standing still you'll get at least 5 pokemon, 1 every 5 minutes if you don't screw anything up especially terribly somehow. (I never get anything in the final minute, so I wonder as to why that is--my theory is that under 2:30 remaining nothing will spawn.) There is not, however, a penalty for going too fast when using incense, there is (I think) only a distance/speed cap as to the effect of the incense, otherwise on the highway a passenger could get many pokemon a minute.
