The construction milestone comes after weeks of overnight lane closures.
Installation of the girders began on April 15, starting on the southbound side of the freeway and prompting daily overnight closures of those lanes. At the beginning of May, the work shifted to the northbound side, prompting nightly closures of those lanes.
Each of the girders used for the project weighs between 126 and 140 tons, stretching from the center median to the freeway shoulder.
Construction on the wildlife crossing began in 2022, and it is expected to be completed by late 2025 or early 2026.
Wildlife crossing in Agoura Hills on track to open by early 2026
While crews have made progress, it will be at least another year until the overpass is finished.
"It's going to be hard for travelers to see much of the work that's going on above the bridge, but there will be a lot of workers day and night constructing the concrete deck," said Michael Comeaux with Caltrans.
The crossing aims to provide connection between the population of mountain lions in the Santa Monica Mountains and other wildlife to the larger and more genetically diverse populations just north of the freeway. It will be the largest of its type in the nation, officials said.
Crews make steady progress on wildlife crossing over 101 Freeway in Agoura Hills
Decades of road construction and development have been deadly for animals trying to cross the area's freeways, while creating islands of habitat that have genetically isolated wildlife ranging from bobcats to birds and lizards.
Wildlife advocates hope the crossing can save the threatened local population of mountain lions from extinction, which could become inevitable if lions continue a historic pattern of inbreeding due to the limited numbers of cats in the area.
City News Service contributed to this report.