> Milton Keynes tried separating pedestrians and cyclists, it is mostly considered a failure

That's not what I was suggesting, I was suggesting restructuring cities so that pedestrians and cyclist have their own road network accessible to residential streets and businesses. This road network would complement artery/busy car roads as opposed to being a sidewalk next to them.

Where I am, cycling on the sidewalk is prohibited but i see people use it all the time, even where is a bike lane because cars don't take enough care, especially if you wear a helmet. There are many "memorial" roadside spots where cyclists and pedestrians were killed by cars. Walking with someone is also hard because even smaller busy roads have loud cars that make it harder to talk. But the one time my family lived close to a walking/biking trail, everyone started taking routine recreational walks (weather permitting)

Most of the reasons you mentioned are easily solvable except the first one, it would indeed require businesses placing signages and retargeting to include foot traffic. But if you think about it, if these complementary roads are parallel to arterial car roads, food trucks and street vendors will flock there. I also see people walking their dogs next to unpleasant busy car roads when they really prefer the type of roads i spoke about, but also dog parks and other recreational human friendly spot could be exclusive to these roads. Milton keynes seems to have had an implementation/planning problem.

> Tunnels are quite dangerous, having cars at high speed for long distances is extra dangerous. Having them leave the tunnel is either a highway junction or a stroad. There is one in Boston though. https://www.youtube.com/watch?v=d5pPKfzzL54

Freeways already have cars at high speeds for long distances. What I am proposing isn't making the tunnels very fast, just slightly faster than existing roads but also actually faster because less pedestrians and stopping. A 30mph road might become 45mph not 60mph. The right lane would be slow enough to take a ramp to/from parking lots.

I was also suggesting the tunnels for artery roads, not huge highways. And unlike boston, there would be no addding lanes, above ground would be converted to pedestrian roads and/or metrorail where it makes sense. This can be done one road section at a time, roads would dip into a tunnel and then dip out. Sidewalks would be replaces with space for offramping. Places with underground parking fit even better. Another huge advantage of this is that there could be much more grass/soil/greens above ground making the city cooler and more flood resilient. Boston replaced a highway but used that to add more lanes. They had their railway add more stations according to that video but if they went my that would be done to begin with above ground where there are cars now