Crews mostly completed a large, concrete wall along the north side of Highway 101 near Agoura Hills this month – one of the initial steps in building a first-of-its-kind wildlife crossing.

Construction started last year on the bridge over the 10-lane freeway, a barrier for mountain lions, mule deer and other wildlife in Ventura and Los Angeles counties. Ten large columns went up in the median over the past several months and work on a second wall started along southbound lanes.

Billed as the world’s largest urban wildlife crossing, the bridge may help save an isolated mountain lion population from extinction. It will stretch 210 feet long and look like natural habitat.

The crossing is expected to be wide enough to fit roughly a dozen traffic lanes if designed for vehicles, said Michael Comeaux, a spokesman for the California Department of Transportation.

During construction, officials lowered the speed limit and narrowed lanes to accommodate work on a short stretch of the 101 near Liberty Canyon. While lane and ramp closures have happened overnight, the freeway so far has remained open in both directions.

That is expected to change over several nights in early 2024. The freeway will shut down in one direction at a time as cranes lift more than 80 long, concrete beams into place. Built in Riverside County, those girders will form a sort of skeleton crossing, stretching from the walls on either side of the freeway to the middle support columns.

Midnight to 4 a.m. closures will be announced in advance and traffic is expected to detour on Agoura Road, Comeaux said.

The work initially was expected to happen this summer but the timeline shifted. The change is not expected to add time to the overall schedule, he said.

When will construction finish?

Caltrans officials say they hope to complete the crossing by late 2025 but caution that is only an estimate.

The first phase is expected to wrap up late next year. But plans for a second phase are still being designed, Comeaux said. The later work will include extending the crossing over Agoura Road and relocating utility lines to an underground tunnel.

Comeaux called the work to move a major water main as well as power and gas lines “a significant part” of the overall project cost. But the actual cost and timeline is not yet known.

“We’re hopeful that it can all be accomplished by the end of 2025,” he said. “At this point, there are still too many unknowns to call it anything more than a target date.”

C. A. Rasmussen Inc., of Valencia, received a $23 million contract to complete the first phase of construction, which includes the bridge over the freeway. More than $100 million in a mix of public funds, private grants and donations have been raised to pay for costs including designs, studies and construction.

How will the crossing help?

The area near Liberty Canyon is one of few places left in the area that has natural habitat on both sides of the 101. The land is also publicly owned and protected from development.

The National Park Service has studied mountain lions in the area since 2002 to see how they’re surviving in an increasingly urban area. They have found mountain lions and other animals approach both sides of the freeway but few attempt to cross it.

The small cougar population in the Santa Monicas Mountains is boxed in by development and highways, obstacles that have led to inbreeding, low genetic diversity and lions killing each other.

Crossings can reduce collisions with wildlife but also add connections to other areas and reduce isolation, experts say.

What is happening with construction?

Crews have started work on the second wall on the south side of the freeway, Comeaux said Friday. The wall goes up in stages with carpenters first building a tall, hollow wooden form, which will be the first half of the future wall.

After steel reinforcements are added next month, crews will pour hundreds of tons of wet concrete into the structure. Then, they will do it all again.

Once the wall is completed, the skeleton structure will be placed over the top.

For live webcams and more information about the crossing, go to 101wildlifecrossing.org. To sign up for weekly construction and traffic updates, email Michael Comeaux at [email protected].

Cheri Carlson covers the environment and county government for the Ventura County Star. Reach her at [email protected] or 805-437-0260.