Undoubtedly one of the clearest explanations I've read.
Why Road-Widening Doesn’t Work ... And What Does, by Adam Greenfield (The Plaza Perspective)
... No longer can road widening be justified as a congestion-easing tool. Road widening lengthens commutes, increases household costs, worsens pollution, harms the economy, and, let us not forget, kills and injures millions of people globally every year. Transportation departments and politicians had the evidence decades ago and many continue to ignore it to this day. We need to make them understand.
The solution to congestion is compact development and multimodal (and sometimes pedestrian-only) streets. Wherever cars come into contact with well-designed human-scaled cities there’ll always be congestion; cars are extremely inefficient uses of space, after all, and are incompatible with great places. The question is: Do we want a lot of traffic congestion or a little?