Both turnstiles are stainless steel and are bidirectional, allowing passengers to enter with fare payment and to exit. A third older type of turnstile, the High Exit Turnstile (HET), is a black-painted unidirectional iron maiden and only turns in the exiting direction. Entrance is also available via Service Entry gates or AutoGates, which cater primarily to handicapped passengers or passengers with large items such as strollers and luggage. These gates double as pushbar Emergency Exits, though they are often used for regular exiting in crowded stations. Lamps At most of the system's entrances and exits, there is a lamppost or two bearing a colored spherical or cube-shaped lamp. These lights roughly indicate the station's availability (i.e. how often it is accessible): green means always a full-time (24/7) entrance and the presence of a 24 hour booth, typically located at the busiest entrance, red means either a part-time booth or no booth, hence either exit-only or more and more often 24/7 entrance with MetroCard exclusively via HEETs. Older lamps are completely colored green or red, while newer ones, called "half-moons", have only the top half colored, while the bottom half is milky white; this is to provide more light, and the half-colored globes have the same meanings as the globes with full colors. There are also some square lamps. The meaning of the lights is poorly understood by passengers and was originally more complicated. Green, yellow, and red lights were introduced in the early 1980s (exactly in 1982) to indicate the entrance's availability, mostly to prevent muggings by warning riders away from entrances that were closed at night. Originally, green signified an entrance located at a full-time station booth, which was open 24/7 and had regular waist-high turnstiles; yellow signified a part-time booth, to which access to the platforms could be gained using High Entry-Exit Turnstiles (HEETs); and red signified an exit-only. This proved too complicated and yellow was dropped in the early 1990s. Red globes now indicate both part-time entrance or exit-only; indeed, a joke when the system was introduced was that "green meant go in, red meant don't. And yellow meant to take a cab." (NYC medallion taxis are yellow). Further, with the introduction of the MetroCard in 1994, the MTA converted many previous exit-only entrances to full-time entrances via HEETs. The introduction of half-colored globes further confused riders of the subway system, and as of a 2002 survey,