LOS ANGELES — You would think that Los Angeles, of all places, would know how to handle a catastrophe.

But in just over 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 for 53 hours, 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.

And they are calling it Carmageddon.

City officials are warning of a traffic nightmare, urging people to stay home or get out of town with pronouncements that have taken on an increasingly alarming tone. “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 Web site, ticking down to the weekend of July 16 and 17.