George B...
Bush fell of the Segway because he had not turned the power on. No power, means no motors or Gyros = no stability.
For those of you who have never ridden one, try it.
Yes a Segway will not get you fit like a bike.
But is it unstable, no, I took photos, two handed standing on a segway - I'd not try that standing on my bike.
Safe for the streets, yes if you can keep idots off of them - like cars, bikes, feet.