I am a city planner with a goal of low-carbon transportation.  In regards to transportation, my first instinct is to make everyone walk or ride their bike, but then I realize that isn’t very practical at all.  Then I go to electric cars for all residents, but that would not only be impractical but also hideously expensive.  I then find my solution.  A system of trains, as clean as I can get them, that run across the city is a waffle pattern, going both north to south and east to west, with one train per road.  The roads would be train tracks (there would be sidewalks, of course); people would either walk, ride bikes, or ride the trains.  Even if one broke down, a person would only have to walk a single block to find the next one.