Since the Keys have only one main road running their entire length, a convenient system of Mile Markers has been developed. The green and white markers (MM) stand about 4 feet tall on the shoulder of the road. They guide travelers from just south of Florida City (MM 126) down the chain of islands to MM 0 at the corner of Whitehead and Fleming streets in downtown Key West.
The markers are frequently used as addresses and directions. They are also a convenient way to differentiate between locations on the "bayside" (also "gulfside") and the "oceanside" of the road. A typical address will be "MM 90 Oceanside," or "MM 90 Bayside." Traveling southwest toward Key West, the Atlantic Ocean is on the left, and the Gulf of Mexico and Florida Bay on the right.