Efficacy and Design of Elevated Road Segments for Small Animal Passage

In response to high road mortality of Yosemite toads over an extended length of road area, a wide crossing structure in the form of an elevated roadway segment (ERS) was placed on top of Sierra National Forest road in 2018. The road is on a flat landscape, with an upland slope on one side and downward slope on the other. Installing passages under the road would have required a significant amount of grading and re-contouring on the upland slope side to make passage entrances accessible. To meet these challenges, the USGS and USFS designed and installed a novel 100-ft long road crossing structure composed of hardwood laminated road mats installed on top of 8-in high support bars perpendicular to the road, allowing for passage of small animals. The passage is permeable to light and rainfall and met specifications for USFS, County, and City roads.
Active-infrared cameras were used to document species activity under and outside of the ERS. After 4 years of data collection, results showed that almost all small animals species that were detected in the forest habitat were also detected under the ERS structure. There were no significant differences in activity of amphibian, reptile and small mammal species among camera sites under the ERS and in the surrounding forest habitat. However, amphibian activity generally trended lower under the ERS than in the forest habitat, while small mammals trended higher underneath the ERS. Due to reduced vegetation cover in the adjacent roadside habitat, we suggest that planting of shrubs adjacent to the ERS may enhance passage for species that avoid open areas. Overall, results showed that the ERS crossing has a high potential to provide increased connectivity for Yosemite toads and a wide range of other amphibian, reptile, and small mammal species.
To meet the need of primary roads and highways, 4 ERS concept designs were developed by transportation engineers that range from low bridge designs to less costly repeating passage designs along a raised roadbed. All were designed so that target species can move along a relatively natural terrain path and cross the roadway with a natural soil bottom similar to the surrounding habitat. All designs have a suggested height of at least 1 foot and wide openings to better ensure high permeability to animal movement. For migrating amphibians, the designs contain sections of grated openings to allow natural light and moisture to reach the passage surface during rainfall events when amphibians typically migrate. Although they may have a higher initial cost, there are advantages in building ERS structures in comparison to below grade crossings, including smaller area of impact, less susceptibility to flooding, and greater suitability in areas with challenging topography (flat lands, hilly and extreme terrains). The designs also eliminate or reduce the need for barrier fencing and can theoretically be built to any length depending upon the needs of the species.