Except that in most cities including New York it's going to be difficult to build something like that in the absence of an existing structure and right of way. The High Line is also in effectively a post-industrial area which is now highly gentrified but wasn't previously exactly prime real estate. And it's still not really located for day to day pedestrian movement as opposed to recreation.
Totally agree, it would be a political and engineering mess to try and implement on an existing busy street. The conditions were very ripe to experiment with the high line.
Not an easy thing to develop certainly, but relative to the cost and level of engineering effort needed to build new subway tunnels, I'm surprised that adding transportation improvements at street-grade and then bumping the street up one level, isn't something that ever gets considered. We could run effective BRT through Manhattan, or surface light rail to solve the crosstown transportation problem.
Imagine how much easier getting from Queens and Brooklyn to the Lincoln and Holland tunnels would be by car or bus if say 39th St, Canal St moved the pedestrians to an elevated walkway. What if Times Square's tourist bottleneck could be eliminated by raising Times Square?
Anyhow, unlikely to happen probably ever, but fun thought excercise
