Source: Texas Transportation Institute. The Urban Mobility Study.
The Travel Time Index (TTI) is the ratio of the travel time during peak hours over the time it takes to undertake the same trip under normal conditions. A value around and above 1 is indicative of recurring congestion levels since the travel time is above the average. TTI is related to 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. 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 TTI has leveled off and has declined in some cases. This is indicative of saturation in vehicle ownership and use.