Picture this: Hundreds of thousands of people are crammed shoulder to shoulder on the Golden Gate Bridge when suddenly the bridge’s gentle arch begins to flatten out. A metal groan then echoes across San Francisco Bay as the majestic towers begin tilting toward each other.

As the towers hit their breaking point, the 3-foot-thick main suspension cables slacken and the roadway splits open, dropping waves of pedestrians more than 200 feet to their deaths.

That almost happened 25 years ago today, at least according to urban legend.

On May 24, 1987, 300,000 people were stuck in human gridlock for hours while getting a rare chance to cross the 1.7-mile bridge en masse on foot to celebrate the bridge’s golden anniversary. Officials quickly closed the bridge, so a half-million other people waiting to cross never got the chance. Still, the enormous, unprecedented weight caused the middle of the bridge to sag 7 feet.

“I’m grateful because if the others had gotten out there, maybe the bridge would have fallen down,” Gary Giacomini, then president of the bridge district’s board, told The Associated Press at the time.

But engineers said afterward that the bridge was never in danger of collapsing. And bridge officials insist that the reason the bridge district isn’t permitting pedestrians to swarm over the Golden Gate for the 75th anniversary Sunday has nothing to do with the threat of collapse and everything to do with the threats of overcrowding and terrorism in a post-9/11 world.

“It’s just not wise,” said Ewa Bauer, chief engineer of the bridge district.

Independent engineers agree with Bauer that structurally the bridge was safe during the Golden Gate’s last big celebration.

“It was probably the biggest load the bridge had ever seen,” said Mark Ketchum, a San Francisco bridge engineer who studied the Golden Gate Bridge from 1989 to 1991. “But it did not exceed the design load capacity of the bridge.”

On fully loaded suspension bridges the size of the Golden Gate, it’s normal to have “deflections” of up to 10 feet, said Greg Deierlein, a Stanford University professor of civil and environmental engineering.

Engineers say suspension bridges are designed to bend and move more than other bridges. The roadway, Ketchum said, hangs from two massive cables that can stretch and swing like a hammock.

In the 1930s, the middle of the bridge was designed to move 16 feet vertically and 27 feet from side to side without causing permanent damage, said Mary Currie, a spokeswoman for the bridge district.

Originally, the bridge was engineered to hold 4,000 pounds for every foot of bridge. And during the mid-1980s, concrete was replaced with a lighter steel framework, boosting that capacity to 5,700 pounds per foot, bridge engineers said during the 50th anniversary festivities.

No one knows the exact weight of the pedestrians on the bridge on that May day. But assuming the average person weighs about 150 pounds and occupies about 2.5 square feet in a crowd, there would have been about 5,400 pounds for every foot in length. That’s more than double the weight of cars in bumper-to-bumper traffic.

The difference between 5,400 and 5,700 pounds might sound too close for comfort. But bridge engineers emphasize that they routinely build in an additional “factor of safety” to account for unexpected loads put on a structure.

The designers of the Golden Gate over-engineered the bridge to accommodate at least an additional 150 percent weight, said chief engineer Bauer.

And even if the crowd 25 years ago had formed a massive human pyramid and exceeded that safety buffer, the deck would “deform” and not break, like a paper clip bending instead of snapping, said Abolhassan Astaneh, a UC Berkeley professor of civil engineering, structural engineering and bridge engineering.

What’s more, even if part of the bridge collapsed, not all of the roadway would have fallen into the bay, Astaneh said. Sections of the roadway are supported by the cables and not by each other, Astaneh said, so if one piece of the deck falls, it doesn’t bring others with it.

“They’re just swings, 50-foot-long swings next to each other,” he said of the roadway’s sections.

Most of the anxiety that day came when the crowds from San Francisco and Marin County met in the middle of the bridge. People were sandwiched together and couldn’t move.

“Then it got kind of scary, because we realized we were trapped,” said Barbara Schnur, a Foster City resident who was 25 at the time. “We were standing there, and then I said to my friend, ‘Dude, this bridge is moving.’ “

Indeed, winds were blowing as fast as 40 miles per hour, so the bridge was swaying from side to side.

“It was almost like you were walking drunk,” said San Francisco native Karl Watanabe, who was 32 at the time. He said he was also worried about the crowd panicking, causing “a mass exodus.”

Luckily, smartphones and Twitter didn’t yet exist, so many of the pedestrians were blissfully unaware the bridge had flattened until they saw the picture in the newspaper the next day.

Engineers say the big screen is the most likely place Californians will see the Golden Gate collapse.

In the movies, the bridge has been a victim of earthquakes, monsters, aliens, mutants, robots, solar radiation and even a megashark.

But in the flicks he has seen, Ketchum said, the bridge doesn’t ever fall apart the way it actually would.

“They almost always show the main span collapsing into the channel, then the towers falling inward into the channel,” he said.

“That’s wrong,” Ketchum added. “If the main span falls, then all that tension in the cables goes away — and the towers bend back.”