Southern California's terrain is marked by mountain ranges and narrow canyons. And that has long been a challenge for transportation officials as they try to move commuters between bedroom communities and job centers.
Some of the region's worst commutes are in those narrow canyon spaces: The 405 Freeway, where the Sepulveda Pass connects the Westside and the San Fernando Valley, and Santa Ana Canyon, where the 91 Freeway connects Orange County and the Inland Empire.
Now, L.A. transit officials are proposing a tunnel under the Sepulveda Pass.
Q: What do we know about the tunnel?
A: The tunnel would include a rail line and toll freeway. Early estimates place the price tag at $7 billion and $9.5 billion.
Officials recently completed a widening of the 405, but that has not done much to ease congestion.
The 405 carries about 300,000 vehicles a day, a number that the Federal Highway Administration expects to grow by nearly 50%, to 447,000, by 2025.
Q: Hasn't a tunnel been discussed along the 91 Freeway?
A: Yes. A decade ago, officials fiercely debated a 11.5-mile tunnel that would have connected Interstate 15 in Corona to the Foothill-Eastern tollway in Irvine. Its most expensive version would cost $8.5 billion, according to estimates.
That tunnel was pushed by Inland Empire officials desperate for relief for commuters who drive from their homes there to jobs in Orange County. But there was strong opposition from property owners and others.
Officials said the tunnel could handle more than 100,000 vehicles a day. But many worried about environmental damage to the Cleveland National Forest.
A decade earlier, officials pitched a different tunnel plan. Several alignments for the road were considered, including one that would have taken it on a more southwesterly course from Cajalco Road, and beneath a different stretch of the national forest, before linking up with the Foothill Transportation Corridor.
It also faced heavy opposition in Orange County.
Q: Tunneling has challenges, right?
Yes. Tunneling under mountains has been done around the world, notably in Europe. In Southern California, it has faced opposition.
Residents in South Pasadena and other communities have been fighting a plan to tunnel a leg of the 710 Freeway.
Planners of California's High Speed Rail system have struggled in planning massive tunnels needed to get the trains through the Angeles National Forest.