Source: ITF Transport Outlook 2017, Chapter 5, Mobility in Cities. Note: A congestion index of 0.5 implies that a driver loses 50% more time during morning peak hours because of congestion. Cities are those of more than 300,000 people, which includes 1,335 cases.
There is a strong relationship between economic development and urban congestion levels. Low-income economies have fewer resources available to provide infrastructures such as highways and public transit systems, which is associated with high congestion levels. As income levels increase, more collective resources are available, which improves the quality and efficiency of urban transport infrastructure, implying that large cities with high-income populations have a transit system
There is also the matter of density and congestion. As the urban setting becomes denser, congestion tends to impair circulation with average driving speed a good indication of congestion. While densities lower than 50,000 people per square km tend to have a limited effect on driving speed, as soon as this threshold is reached, driving speeds substantially decline.