What is the 12 Square Inch Rule?

On December 1, 1991, the National Highway Traffic Safety Administration (NHTSA) made effective the final ruling regarding the 12 Square Inch Rule. This ruling requires that the total luminous lens area for a stop and turn light must not be less than 75 centimeters squared (11.625 square inches) when used on vehicles over 80-inches wide. It must be noted that a reflex reflector ring cannot be counted as square inch lens area. When the reflex ring is subtracted from the total 12-inch lens, it leaves less than the 12 square inches required.