"We can invent all the green technologies we like, but we can't tear down blocks full of drafty old structures and start from scratch - to say nothing of the networks of streets lined with wiring, pipes, and tunnels that might be decades, even centuries old. The problem is especially acute in older cities such as Boston.
So how to improve the cities we've got? The answer: retrofitting."
The ideas suggested here for retrofitting cities include bikes-on-demand, a smart grid, and podcars.