Today’s invention is a mobile skyscraper.
Actually it would be a metal frame tower on rollers. This would be designed to fit within the width eg of Manhattan’s long, north-south avenues.
A small number of such mobile towers would move up and down each avenue, supported on rollers near the sidewalks and leaving a tunnel open on their ground floor so that traffic could pass through unimpeded.
The upper decks would have gangways which would move inwards and outwards automatically to allow access for the inhabitants of conventional skycrapers as the frame towers slowly passed.
These towers would enable people to walk across the street from building to building, without going to street level.
They would also accommodate meeting spaces, retail and recreational facilities (such as gyms).
This would offer comparatively cheap, extra space in cramped environments, but without closing roads or permanently damaging the character of these cities.