As development has sprawled far beyond the mountain ranges that rim the Los Angeles Basin, the roads that cut through the hills have become some of the most vulnerable in the nation to mishap or disaster.
The loss of even a single traffic corridor can cause costly delays -- as the deadly fire that temporarily closed Interstate 5 last weekend proved once again.
Commuters and truckers were faced with finding alternatives to a stretch of freeway that handles nearly a quarter of a million vehicles a day in an area where mountains leave few options.
It is a problem that urban planners agree will only get worse.
"If you have to blame it on anybody, blame it on the Spaniards, on the Conquistadors. They had a bad habit of building a city in a bowl," said Alan E. Pisarski, author of "Commuting in America."
"In our arrogance, we forget topography and geography. Every so often, we're reminded."
Local transportation experts say drastic changes will be needed to accommodate a population that is forecast to increase 60% over the coming decades -- growing from 19.5 million in 2000 to 31.6 million by 2050.
As once remote areas on the other side of mountain ranges turn into major population centers, one of the few options to move more people from valley to valley is to tunnel through the mountains, a costly proposition that has drawn protests from some environmentalists.
Along the I-5 corridor, for instance, 21,000 new homes are planned at Newhall Ranch, with another 23,000 to be built at Tejon Ranch. Both developments would rely on I-5 to move a projected population of 120,000 residents.
The weekend's truck crash, where the I-5 interchange meets the Antelope Valley Freeway, killed three and injured 10.
Although Caltrans engineers were able to reopen the roadway more quickly than they initially expected, the closure over the weekend served as a reminder that the mountain interchange is particularly vulnerable. In addition to the most recent shutdown, the route was hobbled by earthquakes in 1971 and 1994.
Even when Interstate 5 and the Antelope Valley Freeway are wide open, they form one of Southern California's numerous natural "choke points," said Robert Cervero, professor and chairman of UC Berkeley's department of city and regional planning.
Conditions, he said, are further complicated by a transportation network that was built mainly in the 1950s through the 1970s to handle significantly less traffic.
"You have very complex travel patterns," Cervero said. "The lethal combination there is terrain, topography and sprawl."
Pisarski said L.A.'s reliance on freeways is not all that different from other Western cities'. What sets Los Angeles apart is topography that provides few alternatives to its main highways. When something goes wrong on a major freeway, "it really screws things up," Pisarski said. "The systems are increasingly fragile."
One possible solution: tunneling through mountains.
Once thought unfeasible, tunnels are now getting a more serious look. Millions of dollars have been committed to study a tunnel that would cut 12 miles under the Santa Ana Mountains and connect Inland Empire cities to Orange County.
The proposed route is designed to ease chronic congestion on the 91 Freeway, which cuts through Santa Ana Canyon and is a key route from Riverside County into the L.A. Basin. Officials estimate the tunnel could cost up to $8.5 billion.
There also has been discussion about a 23-mile tunnel through the San Gabriel Mountains that would connect the Antelope Valley to Glendale, a proposal that Palmdale officials have explored.
But an early estimate of the cost of that tunnel was $3 billion to $5 billion.
Doug Failing, Caltrans district director for Los Angeles and Ventura counties, said the question was not whether such a tunnel could be engineered but whether it would make sense given the limited funds and competing demands for traffic relief.
"It could be done, [but] the cost would be tremendous," said Failing, adding that the agency was not pursuing the Antelope Valley-to-Glendale tunnel.
Such tunnels would create alternatives to the traditional mountain passes that, for the most part, already have been expanded to allow as many traffic lanes as geography allows.
Although those expansions have added capacity, planners note that they have left key corridors exposed to complete closure with no way for commuters to easily go around.
"The redundancy you like to have in the ideal system is not there," said Pisarski, who has studied commuting for the Transportation Research Board, which advises the federal government.
"If something happens to one facility, it can cause considerable pain," he said.
Pisarski said that by adding onto existing roads instead of building new routes, "you put all your eggs in one basket."