Note: ADA regulations now call for a 70 percent contrast between the triangle and the circle elements of Unisex Door signs, as well as a 70-percent contrast between the circle element and the surface the sign is mounted on. 

Why a white triangle? New regulations will begin to require a contrast between the triangle and circle elements of Unisex Restroom Door signs in California, per section 1115B 6.3 of the 
2009 California Access Compliance Reference Manual:

1115B.6.3. "Unisex sanitary facilities shall be identified by a circle, ¼ inch (6.4 mm) thick and 12 inches (305 mm) in diameter with a ¼ inch (6.4 mm) thick triangle superimposed on the circle and within the 12-inch (305 mm) diameter. The triangle symbol shall contrast with the circle symbol, either light on a dark background or dark on a light background. The circle symbol shall contrast with the door, either light on a dark background or dark on a light background."

ADA Restroom Signs Mounting Location and Height: Where permanent identification signs are provided for rooms and spaces, signs shall be installed on the wall adjacent to the latch side of the door. Where there is no wall space on the latch side, including at double leaf doors, signs shall be placed on the nearest adjacent wall, preferably on the Right. Mounting height shall be 60 inches above the finish floor to the centerline of the sign. Mounting location shall be determined so that a person may approach within 3 inches of signage without encountering protruding objects or standing within the Swing of a door.