Source: Texas Transportation Institute, The Urban Mobility Report.
The Travel Time Index (TTI) is a congestion measure developed by the Texas Transportation Institute and applied to a sample of 101 American cities on a yearly basis since 1982. The TTI is the ratio of the time a trip takes during rush hour over the same trip in regular conditions. A value around and above 1 is indicative of recurring congestion levels. Still, it should be interpreted with caution as it applies to the city-wide area and that congestion tends to occur at specific locations. Additionally, it does not reflect the level of operational efficiency a city may have for its congestion management such as high occupancy lanes or accident management schemes.
The above graph plots all the sample cities for all annual observations (1982-2017), implying that one city has 34 observations. There is a relationship between the urban population and TTI (R2 of 0.44). For several cities, the TTI has deteriorated at a faster rate than the population growth over the sampled period. The sample also reveals that around a threshold of 1 million inhabitants a city starts to face recurring congestion. This threshold must be interpreted with caution since cities have a wide range of local characteristics that may impair (e.g. bridges and tunnels) or improve (e.g. a grid of highways) circulation. Inferring this threshold to other cities around the world is also hazardous since they have different densities and modal preferences. Still, it can be assumed that the population threshold could be relatively similar as, for instance, European and Asian cities have higher population densities but lower density of road transport infrastructures than their American counterparts.