Electric bikes cannot ride on motor power alone. Since 2016, electric bikes powered by a throttle have been phased out.
These days, electric bike motors can only apply power when the rider is turning the pedals as well. If you are free wheeling downhill – that is to say without turning the pedals – then the motor will not be adding its power. The moment you start turning the pedals again, the motor will kick in again.
They also cannot provide motor assistance above 15.5mph. You can ride the bike above 15.5mph, you'll just be doing that with the assistance of gravity and momentum and your gearing alone, not with the motor which will cut out automatically above that speed.