Last night I captured an ES4 as a Bird Charger with plans of testing this. I was able to get the scooter up to 20 mph but no faster on a flat surface. Tried again on a slight downhill and was able to get it up to 22 mph with the battery level at 90%.
Bird limits their scooters to 5 mph once they are captured by a Bird Charger. This glitch completely overrides the charger speed limit.
The scariest part is when you let up off the throttle. The scooter continues to travel at its top speed and the regenerative breaks did not work to slow it down. It was very noticeable during my slight downhill high speed run, not as much on the flat surface.
Here's a screen shot from an app I downloaded to track my speed.
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