When I first read that book, I thought it seemed like a lot of wasted space because of the amount of parallel lanes you would need to achieve highway speeds. Then I started driving and noticed how wide our highways already are.
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.
I don't recall the same setup in the I, Robot stories (but it's been a long time).
We might not have conveyor belts for roads, but we are defiantly living in the future.
To your comment, yes, that is pretty amazing, to have my (our) intelligence augmented in this way. Because just writing it down, and reading it once, is not sufficient to remember it.
It took us a long time to understand the power of writing, and it could be that eventual understanding of the power of search will also have vast implications.
Luckily, Wikipedia has our backs on this, with a convenient list of works containing moving walkways: http://en.wikipedia.org/wiki/Moving_walkway#Science_fiction
Several of Heinlein's other Future History books and stories reference the rolling roads.