A bridge can do a lot more than carry traffic.

It can turn two growing cities into one economy. It can make travel easier, open new opportunities for businesses and become a landmark that people recognize around the world.

As the United States celebrates its 250th birthday, Roads & Bridges asked a simple question: If America had a Mount Rushmore of bridges, which four would make the cut?

There are plenty of worthy candidates. The Chesapeake Bay Bridge-Tunnel stretched across miles of open water. The Hoover Dam Bypass Bridge broke new ground in modern bridge design. California's Bixby Creek Bridge has become one of the country's most photographed spans. Pittsburgh, known as the City of Bridges, is home to several possible candidates, including the Smithfield Street Bridge and any of the Three Sisters Bridges.

But this list isn't just about engineering records or postcard views. It's about the bridges that changed the way Americans live, travel and think about what is possible.

In the spirit of Mount Rushmore, which features four U.S. presidents who represent the birth of the nation in George Washington, the expansion of our country with Thomas Jefferson, the development of the country with Theodore Roosevelt, and the preservation of the union with Abraham Lincoln, our Mount Rushmore of Bridges represents history, innovation, connection and legacy.

History: Old North Bridge

Some of the most important bridges in America aren't the biggest.

Old North Bridge in Concord, Mass., is one of them.

On April 19, 1775, colonial militia met British soldiers there in one of the first battles of the American Revolution. Ralph Waldo Emerson later called it the site of "the shot heard 'round the world," and the bridge has been tied to the nation's founding ever since.

The bridge standing today is a reconstruction, but people still come from around the country to visit the spot where history took a different direction.

It's a reminder that America's story didn't begin with towering bridges or interstate highways. It began at places like this, where ordinary people made extraordinary decisions.

Innovation: Brooklyn Bridge

When the Brooklyn Bridge opened in 1883, it stopped people in their tracks.

Crowds gathered just to see it. Newspapers covered it. Engineers from around the world studied it.

At the time, it was the longest suspension bridge in the world and one of the first to use steel-wire cables on such a large scale. It permanently linked Brooklyn and Manhattan, making it easier for people, goods and businesses to move between the two.

Building it wasn't easy.

Chief engineer John A. Roebling died before construction began. His son, Washington Roebling, took over but later became seriously ill from decompression sickness. Emily Warren Roebling stepped in to oversee much of the work, becoming one of the most important figures behind the project's success.

More than 140 years later, the Brooklyn Bridge is still one of the country's best-known engineering achievements and proof that ambitious ideas can become reality.

Connection: Mackinac Bridge

Some bridges connect more than two pieces of land.

The Mackinac Bridge connected an entire state.

Before it opened in 1957, traveling between Michigan's Upper and Lower Peninsulas meant relying on ferries that could be delayed by rough weather or winter ice.

The nearly 5-mile-long suspension bridge changed that, giving drivers a reliable route across the Straits of Mackinac.

Today, "Mighty Mac" carries millions of vehicles every year, but it's become something more than a transportation link. For many Michigan families, crossing the bridge is part of summer vacations, holiday trips and visits home.

Few bridges have become so closely tied to the identity of the people who use them.

Legacy: Golden Gate Bridge

The Golden Gate Bridge may be America's most recognizable bridge.

When it opened in 1937, it crossed one of the toughest stretches of water engineers had ever tackled. Strong currents, deep water, heavy fog and powerful winds made construction anything but routine.

The finished bridge quickly became famous.

Its International Orange towers have appeared in movies, television shows, travel guides and millions of photographs. Even people who have never visited San Francisco know exactly what it looks like.

Nearly 90 years later, it remains one of the first images that comes to mind when people think about American bridges.

That's the kind of staying power very few structures ever achieve.

Four Bridges Worth Remembering

America has more than 620,000 bridges. Most people cross them without giving them much thought.

But every once in a while, a bridge becomes something bigger than the road it carries. It changes the way people travel. It becomes part of history. Or it becomes so well known that it comes to represent an entire city or even a region.

There will always be debate over which bridges belong on a list like this. Someone will argue for the Chesapeake Bay Bridge-Tunnel. Others might choose the New River Gorge Bridge or the Edmund Pettus Bridge because of its place in the Civil Rights Movement.

That's part of the fun.

But if America were carving a Mount Rushmore for its bridges, Old North Bridge, the Brooklyn Bridge, the Mackinac Bridge and the Golden Gate Bridge would all have a strong case. Each tells a different chapter of the American story, and together they remind us that sometimes the most important landmarks aren't buildings or monuments.

They're the bridges we've been crossing all along.