It also seems like they could do haptics, i.e. superimpose an audio-frequency wave on the motor current. Similar to the steering feedback you get with a lane departure warning system.
But then again it might be difficult/impossible to make it distinguishable from e.g. rough pavement.
This could 100% be done in software -- it's already adjusting the wheel speed a thousand times a second, it would be trivial to add some haptic oscillation.
But then again it might be difficult/impossible to make it distinguishable from e.g. rough pavement.