These are without a doubt my favourite of Asimov's books :)

Earth has evolved to a point where the population lives in Cities (capitalisation is important!) -- massive, hundreds of square miles completely enclosed with no outside areas at all. They might as well be space stations.

Transit within these cities takes place on 'strips' for local transport -- think of them as 'lanes' on a freeway. The first strip you step on to is an acceleration strip (onramp), which gets you up to speed to then step onto faster strips.

There's also an "Expressway", which is basically a metro, which you step onto from the fastest strip (I think there is an acceleration strip too) which takes over when you want to go longer distances.

Then for even longer distances you have aircars (planes!).

It's a really good system, but only for highly, highly urbanised areas. I think there's only 2 or 3 cities on the planet at the moment truly dense enough. New York, and perhaps central London.