When I was researching my 2016 article on the history of the Segway PT I found a source saying the belt-driven prototype code-named 'Sybil' could do 32 km/h (20mph). I suspect that at this speed there is no "headroom" left to prevent the machine from falling forwards (as described by a previous poster).
https://segwaynz.wordpress.com/2016/...ry-first-unit/
Curiously, a linear speed of 32 km/h matches up quite nicely with the motor manufacturer's rated maximum rpm, when driving the i-series wheel via the PT's gearbox.