The impacts of congestion on automobile accessibility. What happens in large European cities?

Every day, a significant part of the population in large cities suffers transport congestion. One effect of this is a change in the spatial distribution of accessibility, which may lead to people or businesses finding themselves temporally in areas where accessibility values are lower than either desired or required. This paper studies changes in automobile accessibility over the course of the day, as caused by congestion of the road network in eight metropolitan areas of the European Union: London, Paris, Madrid, Berlin, Barcelona, Rome, Hamburg and Milan. The study was carried out using millions of data points on real speeds on segments of the road networks gathered over the course of two years from TomTom® devices, which provided for the incorporation of a dynamic perspective of accessibility. In each of the areas studied, the different impacts of congestion on automobile accessibility can be observed from differences in the distribution of opportunities and the provision of infrastructures, as well as from differences in culture and customs. Despite these differences, all cities experience two peaks with a lower value during the morning and afternoon. However, results show differences in the intensity and form of the effects of congestion on accessibility in these metropolitan areas. London, Paris and Rome are the cities where congestion has the greatest impact on automobile accessibility, while the Spanish cities are hardly affected by it.