By comparing rush hour congestion and free-flowing traffic difference, the 75 worst metro areas were identified.
"It was a two-step process, done with data from traffic-tracking firm INRIX, which culls information nationwide from more than 1.5 million GPS units, mostly in freight trucks.
Our first step was ranking the metropolitan areas with the worst rush-hour congestion. The order is based on the peak hour Travel Time Index (TTI) for the metropolitan area each highway is in. TTI is a measure of how much longer it takes to complete a road journey during peak congestion hours compared to free-flow hours. (Peak hours are defined as 6 a.m. to 10a.m., and 3 p.m. to 7 p.m.) Speeds during non-peak hours are used by INRIX to establish this free-flow baseline.
After determining the 75 worst metro areas, we then found the worst highway in each, defined as the most hours of bottleneck congestion, as reported by INRIX. The rankings then provide a still deeper look-at the most congested bottleneck segment for the worst highway in each area."