Possibly commutes are surprisingly wearing because, in addition to the obvious (time taken, polluted air), if I'm a fair sample, the shortest route to anger is a belief that things can and should be different. There you are, in a car on a road, and if there weren't so many people on it, you could just go. If my theory is correct, then commuting in traffic is significantly worse than commuting without much traffic. On the other hand, driving in traffic is more work even without an anger factor. I'm not sure how you'd distinguish the two.