serve as the base for local elevators. This allows architects and engineers to place elevator shafts on top of each other, saving space. Sky lobbies and express elevators take up a significant amount of space, however, and add to the amount of time spent commuting between floors. Other buildings, such as the Petronas Towers, use double-deck elevators, allowing more people to fit in a single elevator, and reaching two floors at every stop. It is possible to use even more than two levels on an elevator, although this has never been done. The main problem with double-deck elevators is that they cause everyone in the elevator to stop when only person on one level needs to get off at a given floor. Buildings with sky lobbies include the World Trade Center, Petronas Twin Towers, Willis Tower and Taipei 101. The 44th-floor sky lobby of the John Hancock Center also featured the first high-rise indoor swimming pool, which remains the highest in the United States. Economic rationale Skyscrapers are usually situated in city centers where the price of land is high. Constructing a skyscraper becomes justified if the price of land is so high that it makes economic sense to build upward as to minimize the cost of the land per the total floor area of a building. Thus the construction of skyscrapers is dictated by economics and results in skyscrapers in a certain part of a large city unless a building code restricts the height of buildings. Skyscrapers are rarely seen in small cities and they are characteristic of large cities, because of the critical importance of high land prices for the construction of skyscrapers. Usually only office, commercial and hotel users can afford the rents in the city center and thus most tenants of skyscrapers are of these classes. Today, skyscrapers are an increasingly common sight where land is expensive, as in the centers of big cities, because they provide such a high ratio of rentable floor space per unit area of land. Another disadvantage of very high skyscrapers is the loss of usable floorspace, as many elevator shafts are needed to enable performant vertical travelling. This led to the introduction of express lifts and sky lobbies where transfer to slower distribution lifts can be done. Environmental impact Constructing a single skyscraper requires large quantities of materials like steel, concrete, and glass, and these materials represent significant embodied energy.