New York City actually did an interesting thing with one of their overpasses (I think it was an elevated railroad). Rather than tear it down, they made it into a park, making it possible to walk about 20 city blocks in midtown on the West Side surrounded by greenery and not having to cross any surface streets.
I've walked it several times and I think it's a great idea. Now many Freepers have this bias against any public money being spent on things that benefit pedestrians but I think it's money well spent. I think motor vehicles are great too but sometimes it's good to just get out in the fresh air and walk around the city without having to constantly make sure you don't get run over by cars.
What fresh air?
You build a park over a freeway or main arterial road where do you think the vehicle exhaust goes?
Vehicle exhaust is hot gas, hot gas is lighter than ambient air temps, it goes up.
Have a nice day while you suck on an exhaust pipe.