Now University of Louisiana Students have made a crawling version of a DIY Segway that is more stable but has a top speed of 3 mph.
One of the more advanced DIY Segways is the balanced scooter which is on version 2 and has a proposed monster segway version.
Version 1 Segway i-Series Version 2 Proposed
Speed 9 MPH 12.5 MPH 15 MPH 30 MPH
Weight 90 lb 80 lb 70 lb
Steering touch pads twist grip handlebars
Version 2 of the Balanced Scooter
How fast can a scooter like this go? There's no fundamental limit. A bigger, heavier one could go highway speed. The danger, of course, is that if something fails it'd be a serious accident. Under 10 mph the rider can probably land on his feet if it falls over, but at higher speed you'd want more protection. Also, the differential steering might be iffy at high speed, and hitting a pothole without any suspension might be hazardous. Someone else with no fear of death should experiment with this.
There are many other DIY Segway projects