Found this for kentucky. You would have to check your state laws.
Personally I don't like them because I want the driver behind me to see my brake lights at all times.
189.055 Brake lights. A person shall not operate any vehicle required by law to be licensed upon a highway unless it is equipped with a mechanical signal device which would indicate an intention to stop or suddenly decrease speed by illuminating at least two (2) red lights on the rear of the vehicle, which are visible from the rear a distance of not less than five hundred (500) feet, unless the vehicle was originally manufactured with only one (1) such red light on the rear of the vehicle.
Effective: July 12, 2006
History: Amended 2006 Ky. Acts ch. 173, sec. 4, effective July 12, 2006. -- Created 1970 Ky. Acts ch. 9, sec. 1.
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