What then is the appropriate role of government with respect to urban spatial structure, from the perspective of the monocentric model? If automobile travel cannot be priced efficiently then government intervention may be warranted to correct the resulting excessive decentralization. Possible policies include second-best cost-benefit analysis of transport projects, minimum density controls, and greenbelts. In fact, policies in the United States have worked in exactly the opposite direction, as emphasized by Downs and others. Subsidies for home ownership, subsidized highway construction and maintenance, fragmentation of local government, and minimum-lot-size zoning are just some of the powerful forces by which government intervention tends to cause more rather than less dispersion in U.S. metropolitan areas. We do not mean to imply, however, that such government policies are the main reason for ongoing decentralization — the phenomenon is far more universal than any particular set of policies.