There seem to be lots of plans (and even some working prototypes) for various autonomous flying cars and flying taxis.

Most of these seem to be based on vertical take-off using some variety of an electric quad-copter (or something very similar).

Conventional planes can glide in the event of engine failure, and traditional single rotor helicopters can auto-rotate. Both techniques should get the aircraft safely (more or less) down assuming the controls remain effective.

What is supposed to happen with a quad-copter based flying car/taxi if the power fails? I don't think the fixed props on a quad copter can auto-rotate, so how are they supposed to land? Ballistic parachutes? or something I've missed?