Runners approve of Golden Gate Bridge lane closures for San Francisco Marathon

SAN FRANCISCO (KGO) -- Terrorism concerns are part of the reason for an unusual closure on the Golden Gate Bridge. The northbound lanes will be closed as runners cross the bridge on Sunday for the San Francisco marathon.

FULL LIST: San Francisco Marathon road closures, neighborhood impact

There are approximately 27,000 entrants, including ultra-marathon runner Dean Karnazas. He's a guy so fit that he ran five miles just to meet us for this interview.

Come Sunday at midnight, Karnazes and 75 others intend to run not one marathon but two, back-to-back, for individual charities.

"We're going to run the run the route in reverse, get to the starting point the next morning when the gun goes off, then run the actual marathon with the rest of the runners," he said.

When they do, and they cross the Golden Gate a second time, the bridge will be a different experience than in previous years.

"There is a cone and you are running and cars whiz by at 55 miles an hour," said Karnazes.

It's a concern shared by the Golden Gate Bridge District. For the first time, they will close the bridge to all northbound traffic from roughly 6 a.m. to 9 a.m.

When asked what they're concerned about, bridge spokesperson Lisa Locati said, "Distracted drivers, impaired drivers, an accident."

The bridge will begin restricting traffic at 1 a.m. Ultimately, the closure might affect as many as 4,500 cars.

But on this day, the marathon runners will outnumber them. For three hours, at least.

For a full list of road closures and how the San Francisco Marathon will impact traffic, click here.