Are you imagining an alternative in which a bike network is added to a city, but not using "existing roads"? Where would these entirely-new roads be placed? Would you knock down the buildings between two car-infrastructure-roads, to pave it for bicycles? I suppose you could remove the car-infrastructure-road entirely and make it just a bike-road / linear park. Or you could put the bike lane underground? Or build a separated-grade elevated parkway network above the city?
You put a sign at the start of the road. "Bike street". This means cars allowed but at bike speeds. You have different paint on the street too, to make it very different from a normal road. I have seen them popping up here and there and it works very well in the towns I bike in.
Are you imagining an alternative in which a bike network is added to a city, but not using "existing roads"? Where would these entirely-new roads be placed? Would you knock down the buildings between two car-infrastructure-roads, to pave it for bicycles? I suppose you could remove the car-infrastructure-road entirely and make it just a bike-road / linear park. Or you could put the bike lane underground? Or build a separated-grade elevated parkway network above the city?