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Photo Credit: Scott Rudel

Q. Where is the bridge underpass being built on State Highway 21?
The wildlife underpass is being built at mile post 18.2. This is just north of the High Bridge and the turn-off to Spring Shores.

Q. How was the location chosen?
Thirty years of data collected on vehicle-wildlife collisions identified several hot spots for accidents and mile post 18.2 is one of them. The site also has several physical attributes that lends itself to building a bridge underpass at this location.

  • Collecting and Analyzing Data: Thirty years of monitoring and telemetry data had been collected on vehicle-wildlife collisions and wildlife movement and migration patterns (i.e. mule deer and elk) in relation to the Hwy 21 corridor, the Boise River Wildlife Management Area and the Boise/Sawtooth Mountains.Analysis of the findings identified important wildlife linkage areas and several hot spots for vehicle-wildlife collisions on Hwy 21. Milepost 18.2 is one of these locations; just up the road at milepost 19.0 is another.
  • Landscape: Findings also indicated the site at milepost 18.2 had several physical and natural landscape features that made the location ideal for a crossing. The location lies at the confluence of two major drainages, Mores Creek and the Boise River at Lucky Peak Reservoir. It also lies at the toe and base of a long mountain ridgeline that stretches all the way to the Sawtooth Mountains. These existing physical topographic features play a key role in how wildlife and particularly big game such as deer and elk move within their home range.Knowing the wildlife movement patterns and the land’s physical and natural attributes made this location an appropriate site for a crossing.
  • Land Ownership: The immediate location of the bridge underpass and the majority of the land in the surround area is managed by the Idaho Fish and Game, Idaho Department of Transportation, U.S. Bureau of Reclamation and the U.S. Forest Service. This facilitates the construction of the underpass and the wildlife fences, and assures these structures will be conserved in perpetuity.

Q. Were construction considerations factored into the location and design?
Yes, construction activities, types/quantities of materials, equipment, labor and associated costs were all factored into what design and location would work best. Another factor was a design that could be built in a single construction phase instead of two phases, which would provide significant cost savings.

Based upon construction and wildlife factors a 75-ft bridge underpass was chosen. As it turns out, the least expensive structure to build matches the best structure for migrating deer and elk.

  • A bridge underpass design could be done in a single phase of construction. All other options considered for the proposed wildlife crossing structure, including any type of culvert or other bridge design (i.e. overpass), would have required two phases of construction.
  • The bridge underpass design could utilize the existing Hwy 21 roadway prism width and roadway vertical and horizontal alignments to permit a single phase of construction.
  • Traffic could be diverted around the construction site of the bridge via a temporary shoe fly.
  • Existing turnouts located immediately north and south of the bridge location provide suitable staging areas for construction use.
  • Finally a bridge underpass at this location best met the requirements of the species in question (mule deer and elk with a secondary emphasis on pronghorn). The location of the roadbed sits approximately 15-20 feet on top of the base of a preexisting natural ridge which has two large open draws that converge at this location, making it an ideal place to fit within the natural topography of the surrounding area. This allows for a minimum 30-foot wide by 15 foot high passage space between the wildlife surface pathway and the bottom of the bridge. The bridge underpass design allows for a larger passage way for animals to use when compared to other comparable designs considered but not chosen, such as a culvert or similar cast in place structure.

Q. How do you know deer and elk will use the underpass?
Although there are no guarantees, based on 30 years of data we know deer, elk and other wildlife already use this location to cross Hwy 21. The key to having animals go under instead of across Hwy 21 is an 8 to 12 foot high woven wire mesh exclusionary big game fence that will run parallel to Hwy 21. It will stretch for 1.0 miles on the west side and 1.65 miles on the east side of the highway. The fence is designed to prevent big game and other wildlife from gaining access to Hwy 21. Instead wildlife will follow the fence line looking for a way to cross, which will lead them to the underpass.

Certain design criteria were used to make the underpass itself compatible with survival instincts of deer, elk and other wildlife. In order to allow deer and elk to see what is on the other side of the underpass, the width of the underpass will be the same size as the width of the bridge/roadway. This allows prey animals to scan for dangers in both directions.

Q. How much is it costing and who’s paying for the bridge?
The estimated cost for the wildlife underpass is $550,000 and the entire fence associated with this crossing is expected to cost around $450,000. Federal stimulus funds are being used for the wildlife underpass and one-half of the fence. The local working group is writing grants and fundraising to collect the money necessary to complete the fence.

Q. Will the fence prevent me from using the surrounding area?
No, gates are being built to facilitate bikers, anglers and others who use the area to continue to have access.