CPTED is based on the theory that many crimes are crime opportunity -- i.e., conditions exist to make committing a crime and leaving the scene easy. Countering this opportunity is possible by designing areas with plenty of light and visibility, designing areas to stimulate high activity, designing areas with a single route for entry and exit, and arranging shrubbery and building design elements in such a way as not to obscure areas conducive to crime. Three principles of CPTED are: (1) access controls such as security guards, single entry/exits, and alarms, (2) surveillance such as electronic cameras and high visibility areas, and (3) territorial reinforcement such as with site locations in areas of high traffic and activity. Gainesville passed a city ordinance requiring all convenience stores to comply with certain CPTED elements, since they are a frequent target of robberies. These requirements included keeping more than one clerk on duty during evening and night shifts, locating check-out counters in the center of the store and visible to the street, keeping shelving and window signs low to allow store interior visibility from the street, and changing money handling methods. A dramatic decrease in convenience store robberies resulted from the implementation of these requirements.