Video infographic from "good.is" about how we can reduce traffic in Los Angeles makes a pretty simple claim, "if just 3% of drivers switched to using public transport or cycling to work then traffic could move up to 15% faster." I am sure that's pretty true. The statistic that is worrying though is 87% of Angeleno's drive to work, meaning there not only a need for a lot of road, but a lot of car parks too.
Thinking about car parks its pretty scary, some people think there are maybe 8 car parks for every single car in The United States, much more conservative estimates say about 500 million, and I assume that does not include the garage at home. Given there are 136 million cars, 110 million trucks, about 1 million buses for a total of 247 million vehicles, its no wonder that there is a fair bit of traffic on the roads.