The freedom and mobility we value become the tyranny of commuting. The result is social alienation and lack of community. For those who don't drive, this alienation is acute because the built environment is hostile to pedestrians. Sprawling suburbs place considerable strain on municipal budgets. According to the Canadian Mortgage and Housing Corporation, single family homes in a typical Canadian suburb require four times as much infrastructure to deliver utility services as duplexes.
Transit systems become less sustainable too.
Peripheral routes in low-density neighbourhoods do not have the ridership to cover the costs of the longer trips and the additional transit vehicles required to service outlying areas. Transit cost figures become skewed as profitable routes in high-density areas subsidize service to the fringes. They are loud and dangerous. They smell bad. Without question, they are the biggest source of urban pollution.