One thing I have noticed about MUPs is that they let you drop your attention more easily. Especially if you have a headwind and it's 50 degrees out and you happen to live in LA and so commute in shorts and a t-shirt. You just let your head drop to looking at the floor to lessen your drag and all the sudden something is coming up infront of you and you have to swerve to avoid it.

Then again the constant vigilance required on roads can be tiring as well.