What Is The Longest Bridge In The Florida Keys?

Birdseye view of the Seven Mile Bridge

One of the most famous places in Florida is the Florida Keys. The keys are an archipelago of small islands connected together by a series of bridges that allow you to drive all the way down to the southernmost point in the United States at the tip of Key West. Which leads to the question: what is the longest bridge in the Florida Keys?

The longest bridge in the Florida Keys is the aptly named Seven Mile Bridge. Clocking in at just under seven miles in length, it connects Knights Key (part of the city of Marathon) with Little Duck Key. Elevated 65 feet above the surface of the water, the Seven Mile Bridge provides fabulous views off of both sides.

How exactly can a bridge more than 7 miles long stand up? When was the bridge built? Is it built with modern technology, or is it an older piece of infrastructure? Is there more than one Seven Mile Bridge in the keys? Continue reading to uncover the answers to these and more questions.  

Seven Mile Bridge: Keeping the Keys Connected

An archipelago extending Southwest into the sea from the tip of the Florida peninsula, the Florida Keys are renowned for their unique and quirky people, beautiful beaches and gorgeous ocean views, incredible sunsets, and a distinctive atmosphere that is hard to find anywhere else.

Because the Florida Keys is a connected series of islands, it has the unique characteristic of being a little different everywhere you go. Marathon has a different vibe than Key West, and both of those feel different from Islamorada or other keys.

The keys also share the unique distinction of being threaded together with a series of bridges to let people drive all the way to Key West. This series of bridges is known as the Overseas Highway, and it is part of Route 1, which stretches from Key West, Florida, to Fort Kent, Maine.

The longest bridge on the Overseas Highway is Seven Mile Bridge. It connects a key known as Knights Key, which is part of the city of Marathon (not to be confused with Marathon Key), to Little Duck Key, a small island featuring some picnic areas and boat ramps. The water between Knights Key and Little Duck Key is sometimes known as the Moser Channel.

What Type of Bridge is the Seven Mile Bridge?

Concrete supporting structure under Seven Mile Bridge

The Seven Mile Bridge is a kind of bridge called a box girder bridge. These bridges generally consist of steel or concrete beams crossed by girders. The whole assembly resembles a box when viewed as a cross-section. You’ve likely seen this kind of bridge on a highway overpass or a section of an elevated train. These bridges are relatively simple to build, but they do require a fair amount of effort to design.

The Seven Mile Bridge consists of 440 individual box girder segments. Each segment is constructed from prestressed concrete, a special kind of reinforced concrete designed to improve the lifespan and strength of the bridge. The bridge rides about 65 feet above the water and has been specially designed to carry fiber optic cabling and fresh water to the Lower Keys and Key West.

When was the Seven Mile Bridge Built in the Keys?

The current Seven Mile Bridge was built in the four years encompassing 1978 to 1982. Constructing a bridge is always a massive project, but constructing a bridge that spans a distance of nearly 7 miles is a much more significant challenge.

Engineers decided that the best way to construct the bridge was to have all the sections precast in concrete and then installed. The prestressed concrete and advanced design techniques used to build the bridge took the ancient design of the box girder bridge and elevated it into a modern engineering marvel.

Aerial view of the Seven Mile Bridge

Remarkable as that is, the history of the bridge goes back much further. The original bridge that spanned the Moser Channel was built by the 20th century railroad tycoon Henry Flagler to bring rail service to Key West. The bridge was a remarkable success until the Labor Day Hurricane of 1935This storm, one of the largest to ever make landfall in the United States, caused massive destruction to the bridge and much of the Keys.

Flagler’s rail company was already in some financial trouble and was unable to afford the massive cost to repair the rail bridge. So instead, they opted to sell the bridge to the State of Florida for the princely sum of $640,000. The state then refurbished the existing railway bridges into a highway that connected Miami and Key West

Unfortunately, while it was a boon to the Keys, the bridge was notorious for being dangerous, and spectacular and disastrous car crashes were not uncommon as motorists tried to pass one another on the narrow bridge.

It became clear that the original bridge would need to be improved, but until the current and most modern Seven Mile Bridge was built, the original highway remained operational. It is still operational now but is restricted to pedestrian and bicycle traffic. Drivers on the new Seven Mile Bridge can see the original bridge running roughly parallel to the new bridge.

Phenomenal Florida Fun Fact: Little Duck Key was originally known as Pacet Key, but also sometimes called Big Money Key. Confusingly, the nearby Ohio Key was once known as Little Duck Key.

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How Much did the 7 Mile Bridge Cost?

Bridge-building is an expensive endeavor no matter where you build. However, building a bridge that will cross a seven-mile gap over the ocean is a complex and costly thing to do even under the best of circumstances. The total cost of constructing the Seven Mile Bridge was about $45 million. That cost is in 1982 dollars, which were a little beefier than the current dollar. The same bridge constructed today would probably cost around $100-$125 million. 

How High is the Seven Mile Bridge?

The Seven-Mile Bridge is set about 65 feet above the surface of the water, which equates to about six stories tall! The bridge was built to this altitude so that maritime traffic could pass below. The original bridge built by Henry Flagler’s railroad company in the early 20th century featured a rotating section of bridge that could open to allow ships to pass through; this, however, created significant traffic problems on the bridge.

Is the Seven Mile Bridge Really 7 miles?

With a name like Seven Mile Bridge, we have to ask: is the bridge really seven miles long? The total length of the Seven Mile Bridge is a mere 6.765 miles, which is technically not seven miles. However, that number rounds up to seven, so we say it’s close enough. Besides, “Seven Mile Bridge” flows off the tongue much more easily. It’s also a more enticing name for tourists and visitors. After all, nobody wants to go visit the 6.765-mile bridge.

Traffic on Seven Mile Bridge in the Florida Keys

How Deep is the Water Under the Seven Mile Bridge?

When we think of the sea, we think of deep and dark waters. Not so in the Keys. The waters around the Florida Keys are generally pretty shallow: after all, the Florida Keys is an archipelago, and waters around islands are usually shallower than waters in the open ocean. So the waters under Seven Mile Bridge are only about 18 to 22 feet deep – this shallow nature helps provide the beautiful blue color of the water.

Is Seven Mile Bridge the Longest Bridge in Florida?

The Seven Mile Bridge is indeed the longest bridge in Florida. Seven miles might not sound like a huge distance, but it is a respectable distance for a bridge to cross. For perspective, airliners cruise at about six and a half miles high. So if you stood the Seven Mile Bridge on end, it would be higher than many cruising jets!

The Seven Mile Bridge is about two and a half miles longer than its nearest Floridian competitor, the Sunshine Skyway Bridge. The Skyway (as it is known) is 4.1 miles long and connects the city of St. Petersburg, which sits on a peninsula, to the main Florida peninsulaThe Sunshine Skyway is about 0.04 miles longer than the St. George Island Bridge, which connects Apalachicola to St. George Island.

The Seven Mile Bridge: Beautiful, Functional, Amazing

Seven Mile Bridge is a phenomenal piece of infrastructure. From the masterfully engineered building materials to the skilled architecture, Seven Mile Bridge is a triumphant work of human ingenuity. The flat, understated nature of the bridge lets one enjoy the natural beauty of the Keys and the ocean without being distracted by cables, support posts, trusses, and other architectural features. When you drive over Seven Mile Bridge, you feel connected to the ocean around you.

Not only is it a triumphant piece of engineering, but the Seven Mile Bridge is also a physical lifeline to the Keys: life in the Lower Keys and Key West relies on the bridge to bring fresh water and fiber-optic cables from the mainland. Much of the Keys economy depends on the bridge to provide easy access to the area for tourists and residents. The next time you ride down to the Keys, take a few minutes and appreciate the bridge. Without it, the Keys would not be the fantastic place they are.

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Ever since I was little I have been a traveler at heart. It all started when I was six years old and my family took a road trip to Alaska. I enjoy visiting new places and revisiting some of the great locations that I have been to already.