John Seabrook, in a New Yorker article from a few years back called The Slow Lane, discusses the problem of traffic and its possible solutions. This article, which focuses on New York City, is bursting with some great information about traffic-engineering, and traffic in general. It includes discussions of congestion pricing, the history and science of the traffic jam, and cellular automata, among many other topics. Here are a couple of facts from the article:
Since 1970, the population of the Unite States has grown by forty per cent, while the number of registered vehicles has increased by nearly a hundred per cent—in other words, cars have proliferated more than twice as fast as people have. During this same period, road capacity increased by six per cent.
Because of the limited space and the dimensions of its grid, the heart of midtown Manhattan can accommodate only nine thousand moving vehicles without succumbing to gridlock.