Make it 3D instead of taking space from cars and I agree.
Cars are dangerous and the best engineering solution is to isolate pedestrian scale speed and mass transfer from multi-ton industrial equipment. So go 3D, give cars their own isolated layer, give pedestrians and slightly faster moving exposed humans their own layer.
This is sort of what Boston tried to do with the big dig[1], where several highways were buried under the city. It ended up costing billions and took 15 years to complete. Also, while the RFK Greenway is nice it is also regularly bisected by busy surface roads and on/off ramps so it sometimes feels more like a grassy median than a real park.
Money well spent. We put excrement underground, might as well do the same for cars.
On a side-note, I was in Boston this summer and it's so much more of an airy and accommodating place than I remember from 10-15 years ago. Lot's of the COVID traffic calming and patios still in place. Awesome, go Boston!
Vehicles also include construction cranes, dump and garbage trucks, etc. Those might have unlimited height, but weight is also a factor.
I'm split between an idea like Tokyo's underground mall areas (which work really well with climate control) and just over-specing several stories above ground for a platform layer that can be level across the whole city and completely free of non-emergency moving vehicles.
Cars are dangerous and the best engineering solution is to isolate pedestrian scale speed and mass transfer from multi-ton industrial equipment. So go 3D, give cars their own isolated layer, give pedestrians and slightly faster moving exposed humans their own layer.