The extensive surveillance network provides live video images of most of the city's major intersections to a command center at police headquarters. These images provide valuable information to dispatchers in routing police cars to the scene of trouble. Installed in 1976, the system initially was used for traffic control, but its use has since been extended to general security. The command center is located on an island in the Seine in a fortress-like structure. To reach the control room it is necessary to pass a guarded gate and a badge checkpoint. The control room is located three stories underground and must be entered through a guarded mantrap with CCTV and electrically operated locks. Inside, observers monitor screens 24 hours a day. Images are transmitted from 140 cameras. The cameras can be operated by remote control to pan, tilt, and zoom. Depending on location, signals are transmitted over video or fiber-optic cables or microwaves. The steady flow of information permits a quick response to incipient traffic jams or emergencies. In addition, the traffic headquarters serves as an information clearinghouse, providing information on traffic conditions to subscribing businesses and other consumers.