Housing Innovation found parking requirements can add up to $36,000 to the cost of a single affordable housing unit – more than the cost of using environmentally-friendly Affordable housing is housing which is deemed affordable to those with a household income at or below the median, as rated by the national government or a local government by a recognized housing affordability index. Most of the literature on affordable housing refers to mortgages and a number of forms that exist along a continuum – from emergency homeless shelters, to transitional housing, to non Globally, it is estimated that 1.6 billion people live in inadequate housing conditions. The majority of that population live in "urban slums" which are highly populated, impoverished residential areas consisting of densely packed housing that lack proper shelter and basic necessities such as clean water, food, hygiene facilities, and electricity. Slums typically form in developing countries as populations migrate from rural to urban areas searching for employment and better living conditions. But due to the higher prices of urban living and a range of other factors such as economic stagnation, migrators find themselves forced to relocate to such slums. The largest slum in the world is Neza-Chalco-Itza in Mexico, housing almost four million people. According to estimates, by 2030 1 in 4 people on the planet will live in a slum or other informal settlement. Meanwhile, there are approximately 140 million people worldwide who are homeless, usually as a result of being evicted from slums. Since slum residents do not own a title or lease to property, they are often evicted from their home by someone with a claim to the land and left without one. The same principle is applied in the United States as people who are unable to afford their mortgage or rent are evicted and left homeless. A number of researchers argue that a shortage of affordable housing – at least in the US – is caused in part by income inequality. David Rodda noted that from 1984 and 1991, the number of quality rental units decreased as the demand for higher quality housing increased. Through gentrification of older neighbourhoods, for example, in East New York, rental prices increased rapidly as landlords found new residents willing to pay higher market rate for housing and left lower income families without rental units. The ad valorem property tax policy combined with rising prices made it difficult or impossible for low income residents to keep pace. Lack of affordable housing places a particular burden on local economies. As well, individual consumers are faced with mortgage arrears and excessive debt and therefore cut back on consumption. A combination of high housing costs and high debt levels contributes to a reduction in savings. These factors can lead to decreased investment in sectors that are essential Globally, it is estimated that 1.6 billion people live in inadequate housing conditions. The majority of that population live in "urban slums" which are highly populated, impoverished residential areas consisting of densely packed housing that lack proper shelter and basic necessities such as clean water, food, hygiene facilities, and electricity. Slums typically form in developing countries as populations migrate from rural to urban areas searching for employment and better living conditions. But due to the higher prices of urban living and a range of other factors such as economic stagnation, migrators find themselves forced to relocate to such slums. The largest slum in the world is Neza-Chalco-Itza in Mexico, housing almost four million people. According to estimates, by 2030 1 in 4 people on the planet will live in a slum or other informal settlement. Meanwhile, there are approximately 140 million people worldwide who are homeless, usually as a result of being evicted from slums. Since slum residents do not own a title or lease to property, they are often evicted from their home by someone with a claim to the land and left without one. The same principle is applied in the United States as people who are unable to afford their mortgage or rent are evicted and left homeless. Lack of affordable housing can make low-cost labor more scarce, and increase demands on transportation systems (as workers travel longer distances between jobs and affordable housing). "Faced with few affordable options, many people attempt to find less expensive housing by buying or renting farther out from their place of employment, but long commutes often result in higher transportation costs that erase any savings on shelter." This has been called the "drive 'til you qualify" approach, which causes far-flung development and forces people to drive longer distances to get to work, to get groceries, to take children to school, or to engage in other activities. A well located dwelling might save significant household travel costs and therefore improve overall family economics, even if the rent is higher than a dwelling in a poorer location. A 2012 study led by Pembina Institute affirmed that the additional expenses of long-distance commuting often negated the savings associated with lower housing costs. In both large metropolitan areas and regional towns where housing prices are high, a lack of affordable housing places local firms at a competitive disadvantage. They are placed under wage pressures as they attempt to decrease the income/housing price gap. Key workers have fewer housing choices if prices rise to non-affordable levels. Variations in affordability of housing between areas may create labour market impediments. Potential workers are discouraged from moving to employment in areas of less affordability. They are also discouraged from migrating to areas of high affordability as the low house prices and rents indicate low capital gain potential and poor employment prospects. Lack of affordable housing can make low-cost labour scarcer (as workers travel longer distances).