Texas Transportation Institute, The Urban Mobility Report.
City Size and Roadway Congestion Index, United States, 1982-2009
The Roadway Congestion Index (RCI) 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 RCI
measures the density of traffic across an urban area in relation to
the overall capacity of the transport system to support it. 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 graphs plots all the sample cities for all annual
observations, implying that one city has 28 observations. It is
clear that for several cities that the RCI 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 comparatively lower density of road transport
infrastructures than their American counterparts.