I've often thought that for dense cities such as New York, and London, a Monorail would be a good solution to add to the public transit system. Picture this: A

I've often thought that for dense cities such as New York, and London, a Monorail would be a good solution to add to the public transit system. Picture this: Along most of the major roads fit very tall archways every few hundred feet, and place the Monorail on-top. As long as traffic below can continue unobstructed, then the two can co-exist. Additionally add direct-into-building entrances/exits at the monorail platform height (2nd or 3rd storey (US buildings called the 'ground' floor '1st', right?) ). Monorail can be driver-less, and run on clean[1] energy. It seems a bit of a no-brainer to me. -- [1] For various values of 'clean'.