LOS ANGELES » The intersection of Interstate 405 and Sunset Boulevard, cutting through a prosperous stretch of rolling Los Angeles hills and estates, is notorious for its knots of traffic and frustrating delays. Traffic is so bad that it is hard to figure out how it could get any worse.

Well, a $1.3 billion highway reconstruction project that began with a blizzard of alarming detour signs the other day is about to make it much worse. And that has put a large part of this city on edge.

The goal would appear simple and even admirable: to add a 10-mile car-pool lane on the 405, among the most reviled and traffic-snarled freeways in Los Angeles, as it approaches and rolls north over the Sepulveda Pass, connecting the city’s west side to the San Fernando Valley. But given the nature of this particular operation — basically open-heart surgery on the central circulatory system of this traffic-obsessed town — it is anything but.

What looms is an alleged three-year marathon of open and closed exit ramps, shut and narrowed lanes, banging overnight construction, detours sending traffic rumbling through some of the city’s most elegant neighborhoods, and a reminder of the price paid for the absence of meaningful public transit. It is all being chronicled in a stream of e-mail alerts and Twitter postings from transportation officials, who are doing what they can to keep everyone calm — with mixed success.

"It’s going to last for three years, but it’s going to take 23 years off my life," said Jake Lawson, a 40-year-old actor who lives in the San Fernando Valley and now adds an hour to his regular trips to Santa Monica for acting auditions. "I’ve begun turning to my favorite radio station, contemporary Christian, so I can pray to the good Lord to just let me get through this as I’m sitting in traffic for three hours."

There is nothing unusual about people in Los Angeles complaining about traffic (it is not as if they would dare complain about the weather). But this construction project has an unusually broad reach, creating not one but two very distinct problems.

It is making the already impossible 405 even more impossible, creating headaches for north-south commuters all along the west side of Los Angeles, from the San Fernando Valley to the South Bay beach towns, not to mention anyone trying to get to the Los Angeles airport, UCLA or the Getty Museum. An estimated 300,000 vehicles use the 405 to cross the Sepulveda Pass every day. (As is the case with most highways in Southern California, this one is referred to simply as "the 405," rather than "Interstate 405" as it would be called in most other states.)

And it is narrowing, and at times closing, three bridges that carry surface streets over the 405, most significantly, the Sunset Boulevard bridge. An e-mail "notice of night construction" from the project managers on Wednesday informed residents that for six days this week, "Sunset Bridge may be intermittently closed between the hours of 9 p.m. and 6 a.m. for equipment delivery."

Sunset is the only remotely convenient way to travel from the exclusive part of town east of the 405 that includes communities like Bel Air and Beverly Hills to the similarly exclusive part of town west of the 405 that includes Brentwood, Pacific Palisades and, eventually, Malibu.

And it is not just commuters. Private elementary schools are considering how they are going to get school buses across another bridge on Mulholland Drive over Interstate 405 (sorry, the 405) in time for morning classes. Supporters of the Getty Museum are worried that visitors will find another museum to visit during the construction snarls.

Businesses anywhere near the 405 are understandably worried. "I was very concerned about what was going on, given the scope and closure," said Reginald Archambault, general manager of the Luxe Hotel on Sunset Boulevard. "But I have to say that so far, it has been less impactful than we had feared, knock on wood."

And in a city where social cachet comes from knowing the latest surface-street shortcut, once isolated and peaceful neighborhoods are enduring an influx of vehicles searching for some new way to get home or to work.

Debbie Nussbaum, who lives in Westwood Hills — basically, ground zero for this project — said a driver lost in her neighborhood the other day went winging by at an excessive speed, clipped a bumper on a parked car and flipped over. Nussbaum said the woman had to be cut from the wreckage, but declined medical treatment and asked only for help in escaping the neighborhood and getting home.

"The commuters in L.A. felt that they really needed an extra lane going north," she said. "Given that I live in the city, I didn’t think we really need it. On many days, we have gridlock through our neighborhood because they were detouring cars through our neighborhood."

She added: "This is just beginning, and there’s like three years to go. They won’t give us the full schedule because they know we’d freak out, so they are like, for the next three months we are doing this."

This project is being financed, in part, with federal stimulus money and is intended to complete a long-held goal: to have a nonstop car-pool lane on the 405 from the 10 on the south side of Santa Monica to the 101 in the Valley.

"If we didn’t do this, it would continue to be a nightmare," said Michael Barbour, who is directing the project for the Metropolitan Transportation Authority. "I think that message has gotten across to the locals. They understand it. They are sucking it up over the next three years, approximately. Some of them don’t see it as much as others."

Barbara Broide, a member of the Westside Neighborhood Council, said she thought most motorists were well aware of what was unfolding. "We know there’s construction, and we know we should stay clear of it," she said. "There really isn’t any choice, is there?"

That said, there has always been a sizable contingent, including the author Joan Didion, who spent much of her adult life in Los Angeles, who question the value of even having a car-pool lane, calling it something that does not translate from the neat theory on an engineers’ drafting table to the complicated arteries of Los Angeles. Lawson clearly agrees with Didion on that.

"That car-pool lane — how many people in Los Angeles have the same schedule?" Lawson asked as he prepared for another expedition across the Sepulveda Pass. "None. The car-pool lane in my opinion just takes another lane of traffic away from everybody."

© 2010 The New York Times Company