Crossing Into New Design & Engineering Territory: Bringing the Wallis Annenberg Wildlife Crossing to Life

In California, at least one mountain lion died each week on roads and highways between 2015 and 2022, according to a study by the UC Davis Road Ecology Center. These roads don’t just present fatality risks—they also act as a habitat barrier that limits lion breeding territories and ultimately weakens the biological diversity of the local population. 

It would seem, then, that humans and more of our technology are the last things these mountain lions need. But a highly engineered wildlife crossing literally packed with technological innovation aims to help restore the mountain lion population of Southern California. 

Enter the Wallis Annenberg Wildlife Crossing 

Wildlife studies conducted by the National Park Service “have shown that habitat loss and fragmentation due to urbanization can have significant impacts on wildlife movement and survival. Mountain lions, which happen to be our last remaining large carnivore in the Santa Monica Mountains, are especially vulnerable to these effects,” explained NPS wildlife biologist Jeff Sikich at a ceremony for the groundbreaking of the Wallis Annenberg Wildlife Crossing in 2022. “Our research has also predicted that within our lifetime, we could have a local extinction without connectivity. So, this wildlife crossing could not have come at a better time. It is truly a game changer. After two decades of research and community building, today's groundbreaking sets a path toward saving our local mountain lions and supporting the diversity of wildlife in this whole region.”

Situated in Liberty Canyon over one of the busiest SoCal freeways, the 101, the wildlife crossing will be the largest in the world and “represents the future of sustainable infrastructure,” said US Senator Alex Padilla during the ceremony. Added Steven Keck, then Caltrans acting director: “This bridge demonstrates that this level of innovation, of partnership, of sustainability, it can be done, and it can be done because we do it together.” 

The inspiration for such teamwork was the famous Los Angeles mountain lion P-22 (such lions are also known as pumas or cougars), which had crossed major highways cutting through the Santa Monica Mountains. While he ended up living for many years in Griffith Park and became a Hollywood star in his own right, he was cut off from potential mates, recounted Beth Pratt, regional executive director for the National Wildlife Federation, in a TED talk years ago. After learning about P-22, Pratt began advocating for a SoCal wildlife crossing and began leading the #SaveLACougars campaign. She is now seeing that dream materialize as she works with project partners to bring the Wallis Annenberg Wildlife Crossing to life. 

Working with those project partners is Robert Rock, lead project designer and CEO of Rock Design Associates (RDA). Rock’s firm is working with Sheik Moinuddin, senior transportation engineer of Caltrans, and his team; along with Pratt, the construction firm C.A. Rasmussen, and other partners and stakeholders. Design News spoke with Rock to learn more about the advanced design and engineering behind the crossing. 

Design & engineering to the rescue

Designing a wildlife crossing is quite different from designing a freeway overpass, so the project team faced several “unique challenges,” Rock tells Design News. “At its core, I think the biggest difference is that you're designing for what's qualified as dead load as opposed to live load,” Rock says. “You're designing a structure that is going to carry a substantial amount of weight in the earth that's on top as well as all the vegetation and the kind of episodic movement of wildlife back and forth as opposed to daily traffic from cars, bikes, and pedestrians. So that comes with a different series of considerations, concerns, and also opportunities.

“You're crossing one of the busiest parts of the US interstate system,” he continues. “We've got 300,000 to 400,000 vehicle trips per day along the 101 in this location, which is a massive amount of traffic. It creates a whole series of fun, technical challenges when it comes to impacts from sound and artificial light. 

“But in addition to that, we're mitigating 125 feet of grade change across the site from the north to south, and we are reconstituting about 13 acres worth of space. It's a pretty substantial undertaking that involves everything from simple manipulation of topography to restoration of previous riparian channels and hydrology all the way down to rebuilding the soil ecology,” he adds.

“There’re a couple other technical challenges. It sits along a curve in the freeway, so we've got sight distances and super elevation along the freeway to deal with. We have utility corridors on the south side of the freeway that we have to accommodate. We've got primary power transmission lines. We've got six other vendors that share space on those poles. And if that wasn't enough, we also have a 30-in. water main that travels through the site.

“But it's the right location for the crossing. It's going to be an incredibly effective restitching of what had been there previously and what has essentially been vacant for the last 100 years. We're restoring a piece of the mountain that was shaved away,” Rock says.

The abutments and columns taking shape on April 15, 2024. BRIAN VAN DER BRUG/CONTRIBUTOR/LOS ANGELES TIMES VIA GETTY IMAGES

Getting started

Because of the busy freeway, the team needed a construction plan that would minimize the impact to commuters and the community.

“One of the key variables early on was discussing how we were going to build the structure itself to minimize the impact on the travel way,” Rock recounts. “Some of the initial design strategies looked at doing a cast-in-place concrete structure over the freeway,” but that would have “been more disruptive to traffic” as the team would have had to “close portions of the freeway for a longer period of time, because you can't pour concrete over the top of moving traffic.”

A better alternative was to precast concrete box girders off site. “The deck of the structure itself, the part that spans from the abutment to the central bent or the central columns and then from there to the north abutment, all of those were created as precast concrete box girders,” Rock explains. “There are 82 of them, 41 on each side, and they are roughly 100 feet long and about 4½ by 4½ feet.”

Rock says the precast box girders are like giant 3 Musketeers bars. “The structural engineers cringe every time that I make this reference,” he says. “The center is a chunk of foam and then there's a concrete shell around it.”

The precast box girders were manufactured at a facility in Perris, CA, while the cast-in-place concrete abutments were being constructed on site. “When those precast units were ready, they were delivered at about a pace of three to four per night. There were nightly closures on half of the freeway while the other half remained open so that we were only impacting things in those off hours. The freeway was opened back up again before anybody had to get on their way with their morning commute,” he says.

The first girder was installed April 16, 2024. CALTRANS

“It says a lot to the successes of the Caltrans construction team as well as the contractor and the fact that we received hardly any complaints from the local community during that entire process,” Rock says. 

A November 1 update posted to YouTube shows Rock along with Beth Pratt and Caltrans engineers Sam Toutounchian and Darwin Vargas standing with other members of the team on the poured-in-place cement deck supported by those precast girders. 

Setting the right stage for nocturnal animals

The concrete itself is special. Because many of the animals that would be crossing the structure are nocturnal, the structure itself had to help mitigate light pollution from vehicles.

“There's a persistent impact to the species that will eventually use this space,” explains Rock. “You have a whole suite of species that are nocturnal, that do all of their movement, their foraging, their mating, at night. If you don't find ways to reduce that impact in the natural environment, you're effectively asking those species to either not be in those spaces or to expend an incredible amount of energy trying to block out a nuisance.”

So, “if you look closely at the structure, what you'll notice is that it's not a conventional kind of gray or off-white color. And that, too, is intentional—it's not purely an aesthetic inclination,” Rock says. “We elected to use an integral pigment in the concrete mix to adjust the color of the structure. I like to refer to it as kind of like a dusty mocha color. The ultimate intent was to reduce the surface reflectivity of the concrete. The reason that's important is that artificial light, when it reflects off a lighter-colored surface, bounces off that surface. And in urban environments with particulate matter in the atmosphere, the wavelengths bouncing off that lighter-colored surface create what's called sky glow, so you would end up having this kind of glowing orb over the top of the freeway.”

Rock says that “making these subtle changes by simply adding a different pigment to the concrete to reduce the amount of white light bouncing off will end up paying dividends in the long run relative to how suitable a habitat we can create adjacent to the roadway.”

Even more light to mitigate

Traffic lights weren’t the only lights the team had to manage. The tunnel underneath the crossing required lighting for driver safety, but such lights couldn’t disturb wildlife. 

Research by Dr. Travis Longcore, adjunct professor and co-chair of the Environmental Science and Engineering Program at the UCLA Institute of the Environment and Sustainability and a member of the RDA design team, revealed the impacts of artificial light on natural environments. “That led us to make design decisions that called for the adjustment of the color temperature of the light being used along the freeway and under the structure in this location,” Rock says. “His research indicates that the 4000–5000 Kelvin range (a blue-white color temperature) is disruptive to wildlife and that adjusting the color temperature to a 1600–1700 Kelvin range (more of an amber hue) is more wildlife friendly.”

Rock’s team worked with the Caltrans electrical engineering team to devise a solution to preserve the safety of the travel way while accommodating wildlife, he says. “The lighting is distributed along the freeway in a manner that first and foremost is focused on the safety of the traveling public,” Rock says. “The light fixtures themselves have been fitted with shielding devices to focus the light down toward the travel way and not out into the adjacent habitat area or upward to create sky glow.”

All girders installed. CALTRANS

Protecting wildlife from sound

Sound is another “deterrent to wildlife,” Rock explains, so the team had to mitigate traffic noise, too.

“In the same respect that light has an impact on nocturnal species, sound has an impact on every species that hears on a different wavelength or frequency than we do as humans,” he says. “We're designing for everything from the mountain lion as the apex predator all the way down to a Western Fence Lizard. Those small species are going to inhabit those areas on either side of the freeway. They may not ever use the crossing itself, but they're part of the ecosystem that sustains the species that will move across. If we can't design to suit the habitat desirability of those smaller species on either side, we reduce our potential for creating this kind of restored ecological slice across the freeway.”

The sound isn’t just from the engines or exhaust of passing cars. It's also from “the friction between the tires and the pavement,” Rock explains. “That sound emanates at like a 3-4 ft elevation off the freeway, and then it cascades down from there. In this particular instance, because the freeway super elevation tilts back to the north, the ambient sound on the north side is in the neighborhood of 90 to 95 decibels and on the south side it's 75 to 80 decibels.”

Concrete sound walls are a typical sight along freeways, as “they are good at attenuating high- and medium-frequency sounds up to like 5, maybe 10 decibels,” Rock says. “But they don't do a lot to arrest low-frequency sounds.”

The team was already strategizing the earthen slopes that would approach the elevated crossing, and to make the grading work, some sort of retaining structure would be required, he says. Instead of employing a standard concrete retaining wall and a conventional soundwall, the RDA team devised a strategy to use a large earthen barrier that could be built to attenuate sound. They brought on an acoustic engineer to “inform the design process,” and he developed design software that could plug into the team’s 3D modeling software, Rock said. 

A construction shot from August 2024 shows the "super elevation of the freeway." CALTRANS

Such collaboration allowed the team to “test the degree to which we were able to effect the reduction in the artificial sound impacts in these areas,” he says. “We are targeting a reduction of around 20 decibels as a result of adjustments to the design approach facilitated by the use of that modeling software and the increased ability to dial in at the frequency level.

“We're trying to arrest those low-frequency sounds,” he continues. “The lower the frequency, the longer the wavelength. So, to attenuate a lower-frequency sound, you have to have more mass that you put between the source of the sound and the receiver. If you're dealing with high-frequency sound, having a small concrete wall may be fine. But if you're designing for a long sine wave at a low frequency, you need to have this larger earthen barrier in place to attenuate that sound.”

The crossing will have “large earthen barriers that will parallel the freeway coming off all four corners of the 101 structure,” Rock says. “It varies based on how it blends back into the natural landscape and also some of the other strategies for habitat restoration on the backside of it.”

More than just soil

The earthen structure isn’t just soil—it’s a geosynthetic reinforced embankment. “What that means is that on the freeway side of that earthen embankment, we will have a structurally reinforced slope composed of gabion baskets. It's a mesh basket with stone inside of it selected to blend with the natural stone in the area. We can quickly increase the height of that build up from the freeway side, but that also plays into some of the sound strategies,” he says. “The back side of that embankment is reinforced differently and will feature a tapestry of pollinator plants that serve the wildlife that will call this place home.”

Planning vegetation down to the genetic level

The wildlife crossing needs to support more than just native fauna—native flora, too, is also critical for restoring the natural corridor.  

“If we're going to be designing something to sustain and enhance the genetic capacity of the California mountain lion, we should definitely be looking through the same set of lenses about our vegetative palette. So, one of the core decisions that the project partnership team made very early on was that we were going to create a project plant nursery specifically for this project. We are actually growing all the plants for the project from locally harvested seed that's all been collected within about a 5-mile radius. It's down to the genetic level. We're reinforcing the genetic population of the Santa Monica Mountains with plant communities.”

Such restoration is particularly challenging in this area as it “was absolutely decimated by the Woolsey fire,” Rock says. “So, we're building back genetic capacity and genetic variability and restoring areas that were affected by wildfire.”

Such work includes soil biology. “It's about what's above the surface and what's below. So, in addition to propagating plants that are at that level of genetic specificity, we've also identified biological hotspots within the soil ecology at some of the locations from where we've collected. We've been propagating the soil biology at the project nursery so that the plants are growing in the medium that will eventually become what's installed on the project site. The nursery is supplying not only the plants to the contractor to install on the project, but also the soil inoculant so that we can restore the natural nutrient cycling capacity of the soil.”

The nursery is also preparing these plants to survive in the wild. In most cases, nurseries “respond to the need to grow plants quickly, and so they use artificial fertilizer and artificial soil media to grow those plants,” Rock explains. The challenge with that approach “is when you install those on a site, the care needed to sustain them goes hand in hand with weaning them off artificial fertilizer or you have to continue to supplement them to get them to establish.”

Instead, the team hopes that by using locally genetic plants and soil, the restored vegetation will be “less needy with respect to artificial inputs from irrigation,” he explains. 

A rendering shows how the crossing could support a range of wildlife. ROCK DESIGN ASSOCIATES AND NWF

Soil sandwich with irrigation to prepare for the future

On top of the cement deck will be several layers to waterproof the structure as well as provide drainage and support the landscape, Caltrans engineer Sam Toutounchian explained in the above video. Next will come the restored soil and vegetation and a carefully engineered irrigation system. 

“In the area over the freeway, we've got a giant green roof that has anywhere between one and four feet of soil that has to have a certain sort of density to it because of the requirements of the dead load on the structure,” Rock explains. “We have to have irrigation to make sure that thinner slice of soil profile with some lightweight aggregate at the bottom can drain effectively so that we don't impact the structure. It also must have a soil moisture content similar to the areas on either side of the grade. There’s a balancing act so we don't end up with just a giant green postage stamp over the freeway and then everything else is brown on either side, right? We want to have a level of soil moisture that's reciprocal across that entire footprint.”

The crossing will use recycled water from the Las Virgenes Municipal Water District. “It is an efficient drip system that we've developed with maintenance and long-term suitability in mind because, as you want to have irrigation to get the plants going, we also want it to be a resource that's available to MRCA [Mountains Recreation & Conservation Authority] as the entity that will manage this entire footprint, that vegetated surface. Should they need supplemental water in the future, or should they want to apply water to that surface if and when there's wildfire again, they will have the capacity to do that.” 

Wildfire remains a reality in this ecosystem. “There's no way to completely arrest the potential for wildfire,” Rock says. But the team is working to “restore a profile that will respond better from wildfire should it ever impact the site.”

The restoration of native species and soil ecology should also allow the ecosystem to fight back against invasive plants such as cheatgrass that provide fuel to wildfire, he says. “The hope is that by having species like that without an intense amount of fuel load, you reduce the intensity when there is a wildfire,” he adds.

Such native plants may also help preserve soil ecology. “What we were able to ascertain in some of the initial site visits with the soil scientists is that the Woolsey fire essentially baked all of the ecology out of the soil surface down almost 16 inches because of how intense the fire was. And so, if we can reduce the intensity of fire the next time it happens by having a more-robust native palette, the hope is that it doesn't have that depth of impact in the soil profile.” (The Woolsey Fire burned nearly "100,000 acres of land managed by several partners, including the National Park Service, California State Parks, the Mountains Recreation and Conservation Authority, and the Santa Monica Mountains Conservancy," according to the NPS.)

A design & engineering project like no other—for now 

Rock says that the team approached the design project a little differently than conventional ones. 

“Instead of looking through a conventional lens where we start with the structure and then figure out all the accoutrements afterward, we looked at this from a performative habitat standpoint first and how the structure and the rest of the mechanics of the site strategy serve the performative layer that comes on top,” he says. The idea was to “look at this almost in reverse of a conventional engineering problem. Obviously, structural support, structural stability, and the ability to bridge these large expanses over the freeway were all huge design variables at the onset. But in each of the steps, the design response was one in support of the performative layer of habitat space."

Rock calls the crossing “a foundational piece for my practice,” he says. “I think more than anything it is the collaborative energy on this project that drives me. . . . It wouldn't be the same sort of special project if I didn't have that level of camaraderie that goes along with working with Sheik as the Caltrans project manager or the persistent and unbelievable amount of energy that Beth Pratt and her team put into this project.

“This is truly a passion project for almost every single person that works on it,” he adds. “You see that in the interaction we have on a regular basis. At the end of the day, we all care about it that much more, and we want to see it succeed. It's a project that I think every single person involved with it will see as a piece of legacy that they left behind.

(L-R) Robert Rock of Rock Design Associates, Beth Pratt of the National Wildlife Federation, Jeff Sikich of the National Park Service, and Efren Cazares of SymbioRoots. JON KOPALOFF/STRINGER/GETTY IMAGES ENTERTAINMENT/GETTY IMAGES NORTH AMERICA

“As much as there's novelty and newness in a lot of different things we're putting into this, I'm hoping that it becomes old hat really quickly because I'm hoping everybody's able to build from this. There's so much potential to continue to enhance and make things better. I want people to use this as a springboard, not as a measuring stick,” he says.

Construction of the crossing is nearing the halfway mark. Rock says that “stage 1 is nearing completion with the superstructure largely completed and the soil and native vegetation to follow. Stage 2 will commence in the new year.” 

The crossing is expected to be completed in 2026. Follow the project's progress here, which currently features three webcams.

Messages left by the team on the structure after girder installation. NWF