I once saw somebody literally red in the face, slamming their hands against the steering wheel, while driving a car, just because they were in bumper to bumper traffic.

Then they revved the car, sped forward like a yard, and slammed on the brakes.

Clearly this person was more inconvenienced than the one who had to slow down for you, but i think it illustrates how frustrated and helpless car drivers can feel when affected by "traffic", all without realizing that they are themselves the traffic.

When you're walking or biking, I think you are much less affected by traffic, hence bicycle messengers.

I've also read somewhere that public transit bus drivers have some of the worst stress of all jobs, because they have so much pressure on them, but so much of their job is outside of their direct control (traffic, passengers, etc.)