At 5 m/s (18 km/h) a 500 Watt motor will slow the bike down with a force of 100 N (ignoring generating efficiency). Assuming a rider plus bike mass of 80 kg (about 785 N), the initial breaking deceleration will be a mere 1/8 g. Furthermore, all the breaking will be on the rear wheel, which is far from ideal.

And what's this about an electric bike with no electric lights?