Source: TomTom Traffic Index. Note: The traffic index is the percentage of additional time a road trip takes due to congestion. It is estimated from base travel time under uncongested conditions within the same metropolitan area.
Road traffic conditions vary substantially across cities with the sample underlining an average congestion level of 32%. This means that it takes on average 32% more time to travel within metropolitan areas due to congestion than under uncongested conditions. The most congested cities tend to have higher densities, but this factor alone is not sufficient to explain the variations. Congestion is not necessarily a function of the level of motorization as many American cities are highly motorized, but have lower congestion levels than their European and Asian counterparts. An important factor is the capacity of the metropolitan area to cope with the growth of motorization, which can be impeded by the existing urban spatial structure, inefficient urban planning, and the lack of capital for investments in infrastructure. Therefore, large metropolitan areas in many developing economies, such as India, Mexico, and Thailand, have among the world’s worst traffic conditions, as their infrastructure did not keep up with the sharp rise in car ownership. Paradoxically, congestion is often an expression of economic growth as rising incomes allow for urban residents to afford an automobile, adding vehicles in an urban road system that was designed for a much lower load level.