Parking garage structures, on the other hand, are unique in the outdoor sector because lighting fixtures are well protected from the elements and mounting height is generally limited by low ceilings. While HID lamps are used for lighting parking garage structures, the low-mounting heights of lighting fixtures require a large number of fixtures in order to meet desired illumination distributions. These conditions favor linear fluorescent fixtures, which held 43% market share of lumen-hours sales in 2013, although metal halide and HPS are also prominent contenders.