> In my experience the driving-behavior part of my brain can run virtually autonomously

It can, but I've heard quite plausible claims in the past [1] that you shouldn't let it - because that's one of the things that kills motorcyclists. Your autopilot brain is looking out for other cars quite effectively - but a motorcycle isn't a car, and can slip through un-noticed if you're mind is engaged elsewhere.

[1] Citation needed, but lacking I'm afraid!