Segway PT x2 Reference Manual page 61

Riding against the Handlebar can cause loss of control,
collisions, falls, and injury.
The speed at which the Speed Limiter pushes the Handlebar back
depends on a variety of factors including riding style, terrain, payload,
whether or not the Beginner Setting is enabled, Battery condition, foot
positioning, and other factors.
Conditions that may cause the Speed Limiter to activate and show
the face icon with the straight mouth along with the Warning icon,
when you are traveling at less than the maximum speed allowed for
your operating mode include:
Riding up a steep hill
Riding on bumpy terrain
Riding with fewer than three Rider Detect Sensors
depressed
The first few seconds after you power on and step on the
Powerbase
Low Battery charge
Batteries that are too hot or too cold
Riding down a hill with fully charged Batteries
Because of regenerative charging, if the Batteries are fully
charged, the Segway PT will activate the Speed Limiter to
avoid overcharging.
WARNING!
NOTE:
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