IAAUP (I am an urban planner). I do not have moderator points to mark this as overrated.
A bit of history lets us know that this idea is not new. Grade separation hit its hey-day in the US in the late 60' and early 70's. It was tried in a couple of ways. In Dallas, for example, pedestrian tunnels in downtown were supposed to be an air-conditioned panacea. In northern cities (eg Milwaukee), pedestrian skyways connecting buildings would keep people out of the cold. All the while cars and trucks would speed by uninhibited at the "street" level.
This did not work. It does not work. These facilities are closed, closing, or diminished. Where they still are used they do not add to the pedestrian experience and certainly do not make driving better.
What these grade-separated facilities do is help is kill downtown activity making for a sterile and lifeless downtown. We are physiologically at ease on the ground with the sky above us. Ask an urban designer - or heck, experience this yourself - there are exterior "room" dimensions that work. Too big a space and we feel exposed. Too small and it feels claustrophobic. Tunnels and skyways are not that ideal.
The real answer is making these dense areas more navigable with "slow" transportation. Pedestrian, bikes, and, yes, scooters, should have the highest priority. Grade separation was tried. It just doesn't work because as pedestrians, we want to be "on the ground".
