2 alternating red green square lights

Object:
lights: 2 square lights alternating between red & green with a moment of no light in-between. "red, off, green off" - repeating at about 60 beats per minute
size: from the distance i saw the lights to entire object could have been much larger, the size of visible lights probably 2 - 4 feet across.
position: held a static position during the time of observation, located ~500 feet above the overpass about 500 feet south of the overpass.
activity: object maintained a constant rhythm. of about 60bpm. there was no altitude fluctuations the object held "extremely" still
could not see the source of propulsion or the entire object.