A wheelchair lift, also known as a platform lift, or vertical platform lift, is a fully powered device designed to raise a wheelchair and its occupant in order to overcome a step or similar vertical barrier. Wheelchair lifts can be installed in homes or businesses and are often added to both private and public vehicles in order to meet accessibility requirements laid out by disability acts. These mobility devices are often installed in homes as an alternative to a stair lift, which only transport a passenger and not his/her wheelchair or mobility scooter. Regulations In the United States, the Americans with Disabilities Act of 1990 (ADA) required that all new mass transit vehicles placed into service after July 1, 1993, be accessible to persons in wheelchairs, and until the 2000s, this requirement was most commonly met by the inclusion of a wheelchair lift. In 1993, there were 29,033 transit buses in the U.S. equipped with wheelchair lifts or ramps, representing 52 percent of all U.S. transit buses. By 2001, this figure had grown to 58,785 buses (although the percentage that were equipped with lifts, as opposed to ramps, is unknown). A number of legal regimes in various countries regulate the use of wheelchair lifts, setting forth standards for the devices and requiring certain kinds of businesses to make parking lots accessible to vehicles bearing the devices. In some instances, accessibility standards have been achieved in legal settlements. For example, in the 2005 case of Dilworth, et al. v. City of Detroit, NO. 2:04- cv-73152 (E.D. Mich. 2005), the defendant city conceded that the ADA and its supporting legislation required the city "to maintain the wheelchair lifts on its buses in operative condition; promptly repair wheelchair lifts if they are damaged or out of order; establish a system of regular and frequent maintenance checks of wheelchair lifts; remove a vehicle from service if the lift is inoperative (with limited exceptions); provide alternative transportation when the lift doesn't work and the next accessible bus is more than 30 minutes away." As mandated by the National Highway Traffic Safety Administration (NHTSA), wheelchair-accessible vans with wheelchair lifts are equipped with a safety lift interlock. Designed to prevent operation of the wheelchair van or wheelchair lift in unsafe situations, the safety interlock will sound an alarm if an unsafe condition exists (e.g., the vehicle attempts to move while the lift is deployed) or prevent the