How to lure wildlife across the nation’s busiest highway

When Highway 101 was built through the Santa Monica Mountains nearly 100 years ago, wildlife was captured on either side – lizards, birds and, famously, mountain lions. This has led to inbreeding and even the threat of extinction.

“We have caused so much damage to life on this planet. I mean, we humans have a lot to answer for,” said Beth Pratt-Bergstron, director of the National Wildlife Federation in California.

For more than a decade, she has advocated for Caltrans to build a bridge to help animals like LA’s famous mountain lion P22 expand their geographic territory.

Now that long-dreamed-of bridge is taking shape.

The Valais Annenberg Crossing will be the largest animal bridge in the world, providing a safe way to cross the Santa Monica Mountains on the highway below.

Caltrans is installing more than 80 large concrete supports called girders to shape the bridge so that it can support both animals crossing the bridge and an entire natural ecosystem on top.

Due to the construction work, the agency is forced to close part of the busy road every week evening until the end of May.

Pratt-Bergstrom describes the bridge as “righting a great injustice.”

“This highway has caused devastating consequences for nature here,” she says. “And we are going to fix this.”

The intersection’s design team has spent years researching and experimenting to create a bridge that all types of wildlife will actually use, despite the 400,000 cars that cross the 101 every day.

The first problem? Noise. Mountain lions, foxes and lizards have more sensitive ears than humans and try to avoid traffic noise.

“There are ambient sounds coming off the highway at 80 to 90 decibels,” said Robert Rock, president and CEO of Rock Design Associates and lead designer of the intersection. “The impact that that has on a natural habitat is quite significant for a whole range of different species.”

Rock says they use two types of walls to block about 20 decibels of highway noise: one made of compacted soil and one made of rocks. They also plan to grow wild grape vines over the bridge walls to absorb more sound.

Secondly, there is the brightness problem. Most animals are active at night, when traffic is calmer. But they are also deterred by street lights and car headlights.

Caltrans Project Manager Sheik Moinuddin says they will use custom lighting on the highway around this project.

“There will be no lights on top,” he says. “And we even shield the soft lighting under the bridge in such a way that it concentrates directly on the sidewalk instead of spreading everywhere.”

The 6,000 tons of concrete poured to create the intersection is also made with a specific pigment that reduces surface reflectance. That means less light from car headlights will reflect on the concrete and illuminate the areas around the intersection.

Finally, the design team wanted the intersection itself to look and feel as natural as possible, even though they are creating that habitat from scratch.

“I call it a green roof on steroids,” says Rock. “It’s a giant green toupee on top of a big piece of concrete.”

They take a piece of the native habitat – starting from the plants above the ground to the microorganisms in the soil – and drop about 55 meters of it on top of the intersection.

A team spent years collecting millions of seeds, fungi and microorganisms, all within a five-mile radius of the crossing. They are already growing the plants – including local sage varieties, buckwheat, milkweed and grasses – in a nearby nursery that will one day be planted on top.

Cultivating native fungi and microorganisms will also prepare the soil to help the plants absorb more water (making them more resilient during wildfires) and connect the habitat below ground.

“This is about restoring the entire ecosystem across the highway,” Rock said.

The design team will use rainwater filtering from the crossing to reconnect a creek that was separated by Highway 101, directing wildlife to the crossing.

“Many of these species navigate based on where the natural ridges or natural draws are,” says Rock. “We’re trying to restore as much of those natural ridge lines and draws as possible.”

The hope is that this crossing will be a catalyst and inspiration for more animal bridges in California and the world.

“If L.A. can do this, if we can build a wildlife crossing over one of the busiest highways in the world, then no one has any excuses,” said Pratt-Bergstrom.

Last year, for the first time, Caltrans flagged 43 locations in California that require wildlife crossings – 11 are in Southern California.

But they are expensive. The agency needs more than $900 million over the next decade to achieve these goals. Last year there was only $69 million in the pipeline.

The Wallis Annenberg Wildlife Crossing is expected to be completed in late 2025 or 2026. Follow the progress of construction via this livestream.