Adam Nagourney / New York Times News Service

LOS ANGELES — Already they are calling it Carmageddon.

In just more than a week, 11 miles of Interstate 405 — the north-south spine of the West Side of Los Angeles, which carries 500,000 cars every weekend over the Sepulveda Pass into the San Fernando Valley — is going to shut down from late Friday night to early Monday morning. No cars, trucks or motorcycles will be allowed, to make way for the latest phase in a $1 billion widening project for a highway that serves as an unhappy second home for commuters during rush hours.

City officials are warning of a traffic nightmare, urging people to stay home or get out of town.

“EXPECT BIG DELAY” reads the warning on electronic billboards on highways and streets from Bakersfield to San Diego. The Metropolitan Transportation Authority has an official “Countdown to the Closure” clock on its website, ticking down to the weekend of July 16 and 17.

The Los Angeles Police Department even asked Lady Gaga to post a warning on her Twitter feed, which has 11.4 million followers.

There is a Carmageddon Facebook page and an all-things Carmageddon website, with maps, videos, a Twitter feed and local businesses offering deals to people who stay home.

Kajon Cermak, the traffic reporter for KCRW in Santa Monica, said she was considering doing what any rational person would do on that weekend: leaving.

“I was talking to my husband, and he said we should get out of here and go to Portland,” Cermak said. “The traffic is going to be bumper to bumper. Everybody is talking about it.”

Hospitals are lining up hotel and dorm rooms for employees. The Los Angeles Emergency Operations Center, a high-tech command post built to manage emergency service responses during a natural disaster, will be in full operation, Mayor Antonio Villaraigosa said.