Source: Texas Transportation Institute. The Urban Mobility Study.
Roadway Congestion Index per Year, Selected American Cities, 1982-2011
The Roadway Congestion Index (RCI) measures the density of traffic across an urban area in relation to the overall capacity of the transport system to support it. A value around and above 1 is indicative of recurring congestion levels since the traffic is above the estimated capacity. RCI is related with the urban population, implying that the larger the population the higher the congestion level. In the 1980s and the 1990s congestion significantly deteriorated in major American cities, with many reaching a RCI above the 1.0 threshold. Major factors linked with this deterioration were related to urban sprawl, a growing fleet of trucks and automobiles and the difficulty to provide additional road infrastructures. Commuters were spending an increasing amount of hours in congestion. However, since the mid 2000s, the RCI has leveled off and has declined in some cases. This is indicative of a saturation in vehicle ownership and use.