Each Wildlife species ranging from small-sized to large-sized, requires different environmental factors to be present in order to successfully travel from one area to another (wildlife movement). While some animal species need light (diurnal) to travel, others prefer to travel in the dark (nocturnal), and can utilize a dark tunnel, or other structure, if it is available.
In recent history, depending on where you live, the use and the benefit of wildlife crossings was not taken into consideration during the planning phase for the construction of new roads (Road built without wildlife crossing structures). An example of this is U.S. Route 101 (101 Freeway) in Los Angeles County. This freeway goes from the City (Downtown Los Angeles) to Ventura County (along the coast) and Agricultural areas. The 101 separates two mountain ranges: the Santa Monica Mountains and the Santa Susanna Mountains, that connect to Los Padres National Forest. These mountain ranges contain most of the wildlife within the region. Specifically, the wildlife finds it difficult to cross highways due to lack of wildlife crossings within the area. Wildlife are forced to use transportation structures such as culverts, pipes, and drainages as a substitute for wildlife crossings, if necessary. The effects of impeded crossings on this highway has caused a lack of genetic diversity among the mountain lion (Puma concolor) and bobcat (lynx rufus), naming few. In 2020, the National Park Service detected the first inbreeding defects on a male mountain lion (testis not descended and a defective tail). The highway has also, become impediment to animal movement and an attempt to cross it resulted road kills and injuries to both motorist and wildlife. To solve these problems a wildlife crossing must be built.
Wildlife crossing structures are designed to increase permeability and habitat connectivity across roads. These structures also reduce motor vehicle collisions with wildlife, consequently reducing the likelihood of injury to persons or damage to motor vehicles. Wildlife crossing structures include underpass tunnels, viaducts, overpasses, amphibian tunnels and culverts. Caltrans and its stakeholders have determined that a wildlife crossing is necessary at Liberty Canyon and the 101 Freeway in the City of Agoura Hills. This will allow wildlife from the southwestern portion of the Santa Monica Mountains to increase genetic variability and to avoid road kills by crossing to the northeastern portion of the mountains, continuing to the Santa Susanna Mountains and Los Padres National Forest. The right wildlife crossing will not only serve pumas and bobcats, but it will also serve other target species such as Coyote (Canis latrans), Western fence lizard (Sceloporus occidentalis), mule deer (Odocoileus hemionus), woodrat (Neotoma fuscipes; Neotoma lepida), wrentit (Chamaea fasciata), and western toad (Anaxyrus boreas).