It's a dead-simple concept that can be applied to everything: public money should only be used for public services. If the private sector is viable, it shouldn't need public money to prop it up.
Public money should fund public transit. No public money for private transport infrastructure.
Public money should fund public schools. No public subsidies for charter and private schools.
Public money should fund public health care. No public funding should be wasted on propping up a wasteful private healthcare industry. ACA wastes so much money buying insurance for people when we could just build public hospitals and public clinics.
It's not that private industry shouldn't exist. It's just that private industry, conceptually, shouldn't need to be propped up by social funding. But currently it is. And it's a tremendous waste of money. Public money should only fund public programs. So simple.
Shared-use paths work best when they're low use and low-speed. Ergo why people will walk, bike and drive in the road on a cul de sac but not on a main stroad.
It's common to have separate sidewalks and bike paths on faster, more commonly used routes, because bikes don't actually mix all that well with pedestrians. It's the same reason we don't make sidewalks wide enough to drive a bus down.
By your logic, public car roads are fine so long as there's a bus that drives down them. Even if 99% of the people on them are in a privately-owned bike or car.
Fair enough, it was an unthought retort to "bikes are private, cars are private, same thing."
I'm against building roads for personal vehicles because it is very expensive. Sidewalks and bike paths are cheap to build, cost nothing to maintain (other than SNIC) and last 30+ years.
I'm also not opposed to building roads for the transport of goods and services, that's why humans have built them for recorded history. I've got nothing against personal vehicles using roads built for trucks anyways (the maintenance cost of one truck on a road is equivalent to a lot of cars); so long as the cars don't impede trucks.
My bigger issue the the building of roads specifically for personal vehicles and the building of free (or under market value) parking alongside roads, increasing their cost.
Also, why wouldn't build bike paths the same width as a bus road? It lets you use the same SNIC fleet on paths and sidewalks as roads, allows emergency vehicles to pass, and provides easier access to path amenity maintenance.