The car as an expected accessory to daily life will seem like an outlandish notion to our immediate descendants. There will still be cars for the foreseeable future, but they will be increasingly viewed as intolerable wastes in a reduced-energy world. This is ironic because there are simply so many cars. The place is lousy with them and they're not likely to disappear from the urban setting even if there is not enough fuel to make them all run. The metroplex could come to resemble an automobile graveyard. I'm not quite sure that even light rail (trolleys) will be commonly available despite its relatively high efficiency (though I hope it will). Cities managed to get pretty big and populous before even bicycles were invented so we shouldn't think that having mass or individual transit is a requirement for continued human civilization and all the marks of culture that spring from city life. Mass transit would be a minor luxury in the future and most of us will be walking or bicycling across town, so expect to see less and less of the outrageously luxurious personal auto. Private motorized transport and air travel may both become exclusively the domain of the wealthy within the lifetimes of those reading this. I despair to think of the psychosis this will induce in the average North American who thinks of the car as essentially a limb that grows from one's biological body on cue upon reaching legal driving age. Though fixed path mass motorized transport (rail) is ridiculously more efficient, there remains the question of whether renewable energy sources will be enough to keep up the metalworking necessary to keep the infrastructure in usable condition and also move the mass along the track.
Limited access motor highways will become relics rather rapidly. Overland movement of people and goods will be increasingly by rail, at least for a while. Though far more efficient than automobile transport, rail will be relatively a lot more expensive than it is now. Mining, smelting and shaping the ore for the infrastructure is energy intensive and I'd love to see it done with the energy gotten from wind and water turbines. Considering the energy and cost necessary to get the U.S. system back to serviceable levels across the continent, generating the power to move the trains along the track is a relative cakewalk. It will be possible to move fairly rapidly over large distances overland, but it probably won't be cheap. Sailing by windpower will likely once again become the most affordable method of long distance travel. Thus access to rail and to a good port are next in importance to the basics of nearness to water and food and functioning local production and commerce, but this may not matter until after a lower energy framework has been established with no further corrective contractions in sight. Until towns and cities sort themselves out in terms of feeding themselves, interurban commerce may be the last of concerns. This puts a place like New York City in an awkward position. As a megacity, its prospects are dim, yet it has the redeeming qualities of a remarkably intact rail system and miles of spectacular deep water port. Its commercial center is also crowded with soon-to-be-useless skyscrapers and it, like big cities everywhere, will still be in the middle of a disintegrating conurbative goo.
A port with easy access to a navigable waterway may be of a lot more use than rail. Wind-powered sailing in wooden ships will be cheaper and less destructive than burning organic compounds to move hundreds of tons of steel and cargo along a rail. This is why I'm not even that hopeful about mass transit via rail having much presence in whatever form the city takes in a lower-energy world. The megalopolis linked by highways is certainly an aberration whose passing we are now witnessing, but perhaps even the relatively humble metropolis that sprawled only a bit with the help of intraurban rail links may also be an impossibility soon. Fixed path transit is an order of magnitude more efficient than automobile (cellular) transit, but motorized transport and its underlying support system of any sort will be harder to maintain in a world with far less available energy. Rail may face the same fate as automobile highways. With reduced reliance or the complete absence of motorized transit the physical size of the city will once again be prescribed by what an average healthy citizen could traverse on foot in a reasonable amount of time. The very largest cities in the future may be no bigger than those of the past, that is to say significantly less than ten miles across, perhaps encompassing a relatively tiny 35 square miles (a square 6 miles on a side or a circle with a radius a little over 3 miles). This would allow for a central district easily reached by walking from the edges, even if going from edge to edge isn't quite as easy. Even the very poorest inhabitant living at the edge of such a place could walk to work or the market downtown in less than an hour's time. These larger cities would surely naturally divide themselves into quarters or neighborhoods wherein a resident could perform most daily activities in Leon Krier's suggested 80 acres.