Overseas Highway

Key Largo to Key West

Florida Keys · Florida · United States

Key measurements

Distance

105.8 miles

Elevation gain

110 ft

Elevation loss

111 ft

High point

28 ft

Low point

-2 ft

Net change

-0 ft (9 ft to 8 ft)

Surface

Paved

Route type

point-to-point

Route Overview

Stretching 105.8 miles along the southern tip of Florida, this point-to-point route follows US-1 across the coral archipelagos of the Florida Keys. Beginning in Key Largo, the road treads a low-slung course across mangrove flats, turquoise ocean channels, and narrow island strips, linking the Upper, Middle, and Lower Keys before ending in Key West.

Elevation change along the route is virtually non-existent, peaking at 27.9 feet and dipping to sea level across dozens of open-water bridge structures. The driving character is defined by linear cruising, ocean vistas on both sides of the tarmac, strict speed enforcement, and variable traffic densities dictated by island settlement patterns.

Key Segments & Geographic Transitions

Upper Keys Commercial Belt — Miles 0.0–11.2

Leaving northern Key Largo, US-1 functions as a divided arterial lined with roadside commerce, marina access roads, and residential cut-throughs. The terrain is flat and heavily vegetated with dense hardwood hammocks and mangrove thickets that screen the surrounding water. Intersection density is high here, with frequent turn lanes and traffic signals managing local access.

Keys Causeway & Open Bridge Spans — Miles 12.1–95.5

The central and longest stretch of the route threads through Islamorada, Marathon, and Big Pine Key, hopping from island to island over expansive concrete causeways. Dense commercial zones periodically give way to long, exposed bridges—most notably the Seven Mile Bridge past Marathon—where the Gulf of Mexico meets the Atlantic Ocean. The landscape transitions from suburban island strips to open turquoise flats, shallow salt basins, and sparse mangrove cays. Crosswinds are a persistent physical presence across the unshielded bridge decks.

Lower Keys Marshlands to Key West — Miles 96.5–104.9

Approaching the terminus, the highway narrows through the salt spray marshes and tidal channels of the Lower Keys before crossing onto Stock Island and entering Key West. Traffic builds rapidly as the roadway transforms into North Roosevelt Boulevard, hugging the northern coastline of the island before terminating near Old Town. The environment shifts abruptly from open marine channels to a dense, historical urban grid.

Seasonal & Environmental Context

The Florida Keys operate under a subtropical maritime climate, dividing the year into a dry season (November through April) and a rainy/hurricane season (June through October). Summer brings extreme humidity, intense sun exposure direct and reflected off the water, and sudden tropical downpours that drop road visibility quickly.

Winter brings moderate temperatures and drier air, though strong seasonal cold fronts can generate powerful, sustained wind gusts across open bridge spans. Marine air deposits a constant, thin layer of salt film across the roadway, particularly on windy days near water level.

Logistics & Services

US-1 is the only continuous highway linking the Florida Keys to the mainland, resulting in a continuous service corridor. Fuel, food, and basic services are abundantly available at regular intervals through major key hubs like Key Largo, Islamorada, Marathon, and Big Pine Key.

Because it is a single lifeline highway, incident-related delays or construction can stall traffic for extended periods with no alternate detours available. Fuel and basic services are available throughout the Keys, though options become more widely spaced between the larger island communities.

Important Considerations

Enforced Speed Zones: Reduced speed zones are strictly monitored, particularly through the Big Pine Key corridor designated for Key Deer protection, where nighttime limits drop significantly. * Bridge Crosswinds: Long, unsheltered spans leave riders fully exposed to strong beam winds off Florida Bay and the Atlantic Ocean. * Traffic Flow: Highly variable traffic speeds caused by sightseers, boat trailers, and turning vehicles require constant spatial awareness, especially near public boat ramps and bridge aprons.