Cars are a very convenient way to get around, and since we're building a big city, we people are likely to want to move around in it a lot. It seems that people want "face time", and the techical availability of telecommuting does little to diminish most people's desire to move about physically in space. It would seem, therefore, that we must have good, flexible transport links. However, cars currently run on petrol, and we're trying to imagine a post-petroleum city, so we need alternatives. Electric cars and biodiesel could help. Biodiesel would probably all have to be imported, seeing as arable land is hardly abundant in the Sahara, and electric cars frequently need to be charged, which takes a long time, and so is likely to be an inconvenience, especially as spare electricity is likely to be scarce at night in a solar-powered economy. Without something like nuclear power plants producing lots of spare heat that can be used to economically electrolyse water, a hydrogen economy for vehicles doesn't look feasible.
Therefore, the best option seems to be guided transport, that moves around on tracks and gets its power directly from the grid. Then the draw on power would most be during the day, instead of at night, if we had electric cars with their batteries being charged overnight. This could be trams or trains, but I think there's another alternative that has more promise; it's called PRT (Personal Rapid Transit), and uses tracks, but in stead of big heavy trains following a schedule, has little driverless taxis and lots of stops. Because the gap between cabs can be as small as a second or two, a PRT system can move just as many passengers around as a train or tram system, and it's more convenient for passengers.
According to this Telegraph report, a PRT system is to be built in Heathrow, to start testing in 2008. (More details and pictures here.)
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