  We analyze the congestion data collected by a GPS device company (TomTom) for
almost 300 urban areas in the world. Using simple scaling arguments and data
fitting we show that congestion during peak hours in large cities grows
essentially as the square root of the population density. This result, at odds
with previous publications showing that gasoline consumption decreases with
density, confirms that density is indeed an important determinant of
congestion, but also that we need urgently a better theoretical understanding
of this phenomena. This incomplete view at the urban level leads thus to the
idea that thinking about density by itself could be very misleading in
congestion studies, and that it is probably more useful to focus on the spatial
redistribution of activities and residences.
