Metro planners designed the system with passenger safety and order maintenance as primary considerations. The open vaulted ceiling design of stations and the limited obstructions on platforms allow few opportunities to conceal criminal activity. Station platforms are built away from station walls to limit vandalism and provide for diffused lighting of the station from recessed lights. Metro's attempts to reduce crime, combined with how the station environments were designed with crime prevention in mind, have contributed to Metro being among the safest and cleanest subway systems in the United States. There are nearly 6,000 video surveillance cameras used across the system to enhance security.