2010 ADA Standards for Accessible Design Pocket Guide
810.6.2 Routes and Destinations.
Lists of stations, routes and destinations served by the station which are located on boarding areas, platforms, or mezzanines shall comply with 703.5. At least one tactile sign identifying the specific station and complying with 703.2 shall be provided on each platform or boarding area. Signs covered by this requirement shall, to the maximum extent practicable, be placed in uniform locations within the system.
EXCEPTION: Where sign space is limited, characters shall not be required to exceed 3 inches (75 mm).