Perhaps the biggest problem in commercial aviation has been the distances the typical passenger has had to travel between the airport and the ultimate destination. Planners and engineers have been very concerned with connecting a city’s downtown with its airport. Proposed solutions have included putting the airport underground in the city center and creating air service comprised of vertical take-off and landing (VTOL) aircraft that would not need extensive airport facilities. VTOL airports, as conceived by some, would not take up more than a city block.