The Mile Marker System
Getting lost in the Keys is almost impossible once you understand the unique address system. Many addresses are simply given as a mile marker (MM) number. The markers are small, green, rectangular signs along the side of the Overseas Highway (U.S. 1). They begin with MM 126, 1 mile south of Florida City, and end with MM 0, in Key West. Keys residents use the abbreviation BS for the bay side of Overseas Highway and OS for the ocean side. From Marathon to Key West, residents may refer to the bay side as the gulf side.