ALISO VIEJO -- Traffic has gotten so bad along the eastern rim of Los Angeles' ever-expanding suburban ring that regional planners are seriously considering the once unthinkable -- an 11-mile tunnel under a mountain range in earthquake country.

The proposal calls for the second-longest road tunnel in the world, a new path between sprawling inland suburbs and Orange County, a suburban bedroom community that has developed into one of Southern California's fastest-growing job centers.

Critics, however, question the logic of building a multibillion-dollar project they dub the "terrible tunnel" in a region so prone to earthquakes that an alternate proposal for a double-decker highway was deemed too dangerous. The tunnel would begin barely a mile from a fault that has produced a 6.0-magnitude earthquake.

"It's absolutely absurd to have a tunnel 700 feet below ground in earthquake country," said Cathryn DeYoung, mayor of Laguna Niguel and a vocal opponent. "I mean, would you want to be in that tunnel?"

Residents have voiced that and other concerns leading up to a mid-November decision by local planners over the proposed tunnel's fate.

Building such a long and challenging project -- ensuring adequate ventilation, preventing groundwater leaks and making it comfortable for claustrophobic drivers of up to 105,000 cars a day -- could cost up to $9 billion and take 25 years.

Transportation officials insist something drastic must be done to deal with crippling congestion between Orange and Riverside counties, which are separated by the 25-mile-long Santa Ana Mountains.

Traffic between the counties has exploded over the past two decades. Nearly 400,000 people commute into Orange County daily from four surrounding counties, including Los Angeles, and nearly all of them drive. Even five years ago, nearly 52,000 commuters a day came in from Riverside County, where homes are more affordable, according to the census.

The only major highway connecting Riverside County to jobs-rich northern Orange County carries 268,000 cars a day -- nearly 50,000 more than it was built to handle. Officials expect that total to increase over the next 25 years to nearly a half-million cars per day.

If nothing changes, State Highway 91 will need up to 12 additional lanes by 2030 to handle the traffic between the counties, said H. Tony Rahimian, a consultant who helped devise the tunnel proposal.

Property manager Howard Gottesman, 44, can spend 1 1/2 hours on Highway 91 to travel six miles from his job in Orange County to his home in Corona, just inside Riverside County.

"I call it the longest six miles in the world. It's wear and tear on the car and it's wear and tear on me," said Gottesman. "They need to do something, whether it's double-decking the freeway or tunneling under the mountains. We need relief."

Planners with Orange and Riverside counties have spent the past 18 months and $15 million in federal funds studying just how to get that relief. A 13-member committee made up of county, city and toll road officials is expected to choose elements from three main alternatives by Friday. Two of those alternatives include a version of the tunnel.

Selecting an option with a tunnel would trigger a series of studies and state and federal approvals that could take up to 20 years.

As currently conceived, the four- or six-lane tunnel would make up more than two-thirds of a 15-mile east-west corridor connecting Interstate 15 with two toll roads in Orange County's center. The four-lane option would include a toll, while the six-lane option would be free.

The tunnel would rank second in length to Norway's 15-mile Laerdal Tunnel, which opened in 2000, said Michael Litschi, spokesman for the Orange County Transportation Authority. The California tunnel would handle far more traffic than the Norwegian tunnel, which averages about 1,000 cars per day.

Litschi said engineers were waiting to see whether the committee chooses the tunnel option before doing more studies on the Lake Elsinore fault, but acknowledged that seismic activity is a major concern. Local officials have worked closely with the British company Halcrow, which has helped build some of the largest tunnels in the world and has concluded that the tunnel is "viable and feasible," according to Rahimian, the consultant.

"A tunnel is actually a very safe place. We don't want to run it through the faults and we're going to avoid that," he said. "If they can do it in Europe, then we can do it here."

Many critics, however, say a tunnel will never suffice as the suburbs continue their march eastward into the desert. They suggest more mass transit, including more frequent commuter trains between Orange and Riverside counties.

"Every study shows that you can't build your way out of congestion," said Karl Warkomski, mayor of Aliso Viejo, in southern Orange County. "Eventually, you're going to get a point where you're back to square one -- where we are now, or even worse."

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