One of the reasons the Segway was so expensive is they built it like medical devices, with tons of redundancies in its systems for this exact reason. So if one computer glitched out it wouldn’t just chuck its passenger head over heels.

As we know, once hoverboards got really popular the cheap ones (perhaps most?) were NOT built that way. I saw someone nearly faceplant when theirs ran out of battery and stopped on a dime.

My ninebot scooter (maybe not what you're asking, but it's a Segway product) slows down until it can't carry you anymore. First it just can no longer achieve the maximum velocity, then soon it can no longer reach standard speed, and then you're limping along at 4mph and you can no longer climb any grade without pushing. Finally you're pushing.

It's not particularly usable as a scooter without the battery.

The Segway has redundant control circuitry, motor windings, and battery. If there is a failure of any of these I believe it will fault and perform a controlled stop.

Since we’ve not heard stories of it being involved in deaths I assume that the battery stops providing propulsion score it stops providing gyro. (And depending on how the gyro is designed it might keep spinning long enough for it to slow down and stop.)

> What happens to a Segway when it runs out of battery?

A Segway needs electricity to balance itself, so my guess is that when it gets close to running out of battery, it stops by itself and refuses to continue.