Global Traffic Congestion Index
The navigation and mapping company TomTom sits atop a huge mountain of GPS data from its users. It decided to use its 2014 data to measure the impact of traffic congestions in over 200 cities worldwide.
In the TomTom Traffic Index congestion levels are measured as a percentage of the additional travel time compared to normal traffic (or Free Flow situation). So for instance, Istanbul's congestion level during evening peak hours of 109% means that your 30 minute commute is going to take you over an hour.
Congestion levels mapped for small and large cities
Aggregating the city data produces country averages. The chart below shows the average morning and evening peaks for a selection of countries (only featuring countries with 3 or more cities in the Traffic Index).
Number of cities - sorted by size
Chart shows the number of large and small cities in the worst 50 cities in the dataset, based on overall congestion level. While the entire dataset of 218 cities consists for 33% of small cities, only 17% of the worst 50 cities are small. Source: TomTom
Evening peak congestion levels
Large v Small
On average, congestion levels are a bit lower in smaller cities than in larger cities. Large cities also disproportionately feature in the top 50 of worst congested cities: while large cities make up 67% of the entire dataset, they account for 83% of the top 50.