– America's most iconic bridge is trading plastic pylons for a heavy-duty zipper — and it's shutting down this weekend for the upgrade.

Two years after eliminating human toll takers, the Golden Gate Bridge will replace pylon-toting people with giant yellow "zipper trucks" that will move a heavy, concrete median to reconfigure traffic flow during the peak commute.

The 52-hour closure is the longest for the bridge since it opened in 1937. "It's been closed a few hours for high winds and for ceremonies, but it has never been closed for one full day much less two whole days," said bridge spokeswoman Priya Clemens.

The movable median is one in a string of significant changes to the historic structure that welcomes 40 million motorists a year. Not only were toll booths shuttered in 2013 in favor of automation, but the installation of nets under the bridge to discourage suicide jumpers is expected soon.

The foot-thick, solid median is necessary to improve safety on the bridge, Clemens said, especially to reduce the chance of head-on collisions on the six-lane span. Thirty-six people have died in car accidents on the bridge since 1971 — the last in 2001 — including 16 in head-on crashes. Roughly two dozen people jump from the bridge each year, some 1,500 since it opened.

Still, the change is bittersweet for the lane workers who for decades have "pulled and plugged" the plastic tubes in and out of one-inch sockets while riding in the back of a truck. They have braved torrential rains and bone-chilling fog — starting at 4:30 a.m. — to reconfigure the six-lane, 1.7-mile span into four southbound and two northbound lanes. By 10 a.m., they revert it to three in each direction.

"It's quite an adrenaline rush," said Bill San Gregory, 47, the bridge service operator who will pull out the plastic tubes for the last time. "It's scary yet exciting."

There's an art to pulling and plugging each hole without a miss with no need to push the buzzer to alert the driver to stop and go back. A perfect run is called a "no-hitter."

The project is costing $30.3 million, including the purchase of the two 52-foot-long "zipper" trucks. The massive trucks swallow up a chain of 800, 32-inch-high concrete blocks into a channel in the truck's undercarriage and slides them one-by-one like a zipper one lane over.

The thickness of the barrier will also mean a loss of six inches to the two inner traffic lanes. To allow drivers to adjust, the speed limit will be cut from 55 mph to 45 mph.