These subsidies, however, have not always lead to the intended results because of difficulties with accurately identifying the target population, potential abuse of the subsidy, and large errors of either exclusion or inclusion of the target population. Therefore, as cities grow and their roadways cannot be expanded anymore, the volume of urban traffic needs to be managed within the given roadway capacity. Any surpassing of that capacity leads to traffic congestion - the most apparent transport externality. Furthermore, the extra fuel burned causes unnecessary release of carbon dioxide, hence adding to the overall bill issued by congestion that societies have to pay in the end.