What is essential about these is that they have the potential to put a value on urban amenities and the benefits of agglomeration. They can therefore discourage greenfield development while raising revenue for urban services that can help further mitigate the negative externalities of agglomeration, such as public transport to reduce congestion and green spaces to mitigate higher densities. Many cities in the OECD, particularly in North America, impose development fees on new developments to cover the costs of new infrastructure.