Congested urban transportation networks and emission paradoxes

In this paper, we identify three distinct paradoxical phenomena that can occur in congested urban transportation networks as regards the total emissions generated, which demonstrate that so-called ‘improvements’ to the transportation network may result in increases in total emissions generated. In particular, we illustrate, through specific examples, the following: (1) the addition of a road may result in an increase in total emissions with no change in travel demand, (2) the total emissions may increase with a decrease in travel demand and (3) the improvement of a road in terms of travel cost may result in an increase in total emissions without a change in the travel demand. These examples demonstrate that the network topology, cost structure, as well as the travel demand structure must be taken into consideration in any policy system aimed towards the reduction of emissions due to motor vehicles.