"To determine America's Most Congested Cities, Inrix collects data from nearly 800,000 anonymous, GPS-equipped commercial vehicles that report their speed and location continually to Inrix. The company then processes and blends other relevant traffic-related data such as road sensors, toll tags, traffic-incident data and other resources to figure out when, why and for how long particular roads experience bottlenecking."
"The worst hour for travel in Los Angeles, on average, is Thursday at 5 p.m. In New York, the second-worst city for congestion in the country, it's Friday at 5 p.m. Like L.A., Chicago and Washington are at their worst on Thursdays at 5 p.m., while Dallas, like New York, is most congested on Fridays at 5 p.m."
"But the devil really isn't in the details; it's unsurprising that these cities come out on top when it comes to congestion. The top four spots on the list also have the largest populations in the country: Los Angeles--12.9 million; New York--18.8 million; Chicago--9.5 million; and Dallas--6.2 million."
"What is surprising, however, is that every single city on our list of 20 saw a decrease in congestion over the 2008 calendar year. Some of the cities with the largest decreases in traffic include Riverside, Calif.--which saw congestion drop by 57% --and Detroit, where congestion was whittled by 47%."