The sweeping, graceful arches of Southern California's towering interchanges form some of the most iconic features of the world's most famous freeway network.
But in a region crisscrossed by fault lines, the ramps that soar hundreds of feet above traffic, and the lanes that run beneath them, can be disconcerting territory for drivers hyper-aware of earthquake risks.
Noel Vasquez of Whittier nervously eyes the carpool lane that curves and tilts high above the Harbor Freeway before connecting with the 105 freeway.
"You see it looming, and as you get closer, it just gets taller and taller," Vasquez said. "You drive by and you think, 'Man, I'd hate for that thing to break.'"
Engineers say the anxiety is understandable because earthquakes tend to hit California's freeways hard. During the 1989 Loma Prieta earthquake, a double-decked portion of Interstate 880 collapsed in Oakland, killing 42 people. In 1971, during the magnitude 6.7 San Fernando temblor, ramps linking the 5 and 14 freeways in the Newhall Pass collapsed. The interchange reopened two years later, only to break apart again during the Northridge quake in 1994.
Since 1971, Caltrans has spent more than $13 billion to reinforce the state's bridges and interchanges. Engineers recently finished a years-long campaign to strengthen tall, flexible bridges and make short, strong spans more elastic. In Los Angeles County alone, Caltrans has reinforced 555 state-owned bridges, and local agencies have retrofitted about 280 more.
Despite the precarious heights, giant interchanges are safer in an earthquake than drivers may think. That includes some of the region's best-known junctures, such as downtown's four-level interchange and the elevated East Los Angeles ramps that connect the 710 freeway with the 5 freeway.
"Tall bridges are more flexible," said Tom Ostrom, the chief of Caltrans' earthquake engineering office. "They have the potential to move around more during an earthquake, but they can also absorb more movement."
At the downtown four-level and other major interchanges, crews have encased support columns in steel jackets to prevent the concrete from cracking and falling off. Concrete can lose its strength when it fractures and breaks apart because of seismic pulling or twisting.
Meaghan Murphy, 46, drives from downtown L.A. to Altadena to hike with her Chihuahua, Frank. She said the four-level always makes her wonder: What if?
"There's three layers on top of you, so you have three chances to get squished," Murphy said. "If we had some kind of freeway Tetris situation, my little car wouldn't fare so well. It's not a pleasant thought."
To that, Ostrom says: Don't worry. "These columns are very, very tough," he said. "We're done with work on all the bridges that we believe are the most vulnerable."
Shorter bridges generally don't induce the same degree of driver anxiety. But those spans across rail yards, streets and rivers tend to be more brittle, like the portion of the 10 Freeway that collapsed onto La Cienega Boulevard in 1994. To add more flexibility, crews have added deeper underground support columns designed to make the bridges "act taller," Ostrom said.
The bridge and overpass upgrades don't guarantee freeways will survive unscathed in an earthquake. In every major modern California temblor, freeways have sustained significant damage. Still, improvements made over the last 20 years have made engineers "very confident" freeways will not collapse, Ostrom said.
"The goal is to keep things from coming down immediately," he said. "Some older bridges that are near an epicenter may have to be torn down afterward, but they won't collapse."
Newer projects, including the widened 405 Freeway, are strong and flexible, Ostrom said, and can withstand increased twisting during a major earthquake. Joints now allow for more movement. And extra steel mesh and pipes have been incorporated into designs to prevent spans from crashing to the ground, even if primary support columns fail.
"We used to think that bridges just needed to be very strong, but it's pretty hard to compete with the magnitude that we face here," Ostrom said. "We've changed our philosophy to make bridges that are very flexible. They'll move quite a bit."
Drivers who are caught on or near a high connecting ramp or bridge during an earthquake should wait and stay calm.
When the shaking stops, if there is visible damage, drivers should stay in the car. "You're much more vulnerable when you leave your vehicle," Ostrom said.
If no cracks or holes are visible, he said, drivers on the bridge should continue forward at a cautious pace.
Of course, if you're not on an elevated span when a quake hits, you should avoid them until officials have time to complete safety inspections.
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