One of the most famous bridges in the world is the Golden Gate Bridge that spans San Francisco Bay. Because it was built in 1937, it has become one of the first sights people see if they are coming to San Francisco by ship. The main part of the Golden Gate Bridge is a suspension span. It hangs from steel cables that are supported by two huge towers, one at each end of the bridge. The suspension span was the longest in the world when it was finished in 1937. It measures 4,200 feet between the towers. The floor of thebridge is wide enough for six lanes of cars, three in each direction. There are sidewalks, too, but not many people use them. It costs a lot of time to go on foot from one end of the bridge to the other.The years since 1950 have been called the era of the giant bridge. Most of them have been built in the United States. The Mackinac Bridge in upper Michigan was finished in 1958. It is a suspension bridge, with foundations resting on rock 200 feet below the surface of the water. With a span of 3,800 feet, it was the second in length to the Golden Gate Bridge.In 1964 the longest and heaviest suspension bridge was completed in New York City. Known as the Verrazano--Narrows Bridge, it carries traffic between Staten Island and Brooklyn, parts of the city of New York. Before it was built, commuters to the city had to travel by ferry. Now millions of cars pass over the bridge every year. When a lower deck is added, 48 million cars will be able to cross the bridge in a single year. How many cars will be able to cross the bridge in a single year when a lower deck is added