The best things about a subway system are that sharing one engine between many people is efficient, and that a rail prevents the need for any steering. The worst parts are that tunneling is expensive and stations are even more expensive.

So let's make a new system where everyone uses a fully separate vehicle, relying on advanced software for steering, still needing tunnels, and needing even more vertical access ways onto the street. Also it's a fire hazard, relies on individuals separately maintaining vehicles for safety, needs more maintenance, and dumps a bunch of road traffic onto city streets where it'll still be a problem.