Nevertheless, because of the artificial scarcity of land created by the withdrawal of this commodity from the market for speculative purposes, land speculation is another major factor contributing to the increase in urban land prices. This is particularly true of countries where small, powerful groups own or acquire the best urban land and, through oligopolistic marketing and pricing practices, further interfere with the free operation of the economic laws of supply and demand. These excessive land costs, taken together with the wasteful disuse of infrastructure which passes through land speculatively withheld from the market, comprise a steadily rising share of total housing costs, costs which are also on the ascendancy because of increased building and materials costs and the sometimes inordinate effective demand that exists for scarce housing. Because of these costs and the ability of commerce and luxury housing to outbid government-sponsored housing for prime urban real estate, low-income housing projects, such as sites and services, are usually located on the outermost edges of cities, far from urban services and downtown places of work, where the occupants lead geographically and socio-economically segregated lives.