It's not necisarily a "stressor" if self-driving cars behave somewhat consistently, it's just another signal you can use when making decisions on the road.

"Will this car suddenly decide to change lanes?", "Not likely, it's self driving and there aren't exits or major changes in traffic in front of it".

There will also be quarks in any automatic system. Learning and then predicting these will make the streets safer and more comfortable. For example, perhaps self-driving cars are overly cautions around some local crosswalks. If I'm behind one of these things in the winter I might be aware I should leave even more extra room for the sudden slow downs that other cars will be less likely to do. If I'm smart this wont be the difference between an accident or not, but it will make for a smoother ride.

My larger question is, will proliferation of AI cars increase or decrease net traffic flow. People seem to be driving larger and larger cars slower and slower anyway, so maybe this target is achievable, but this all worries me.