Targhee Pass Environmental Assessment


ITD is now initiating an environmental assessment (EA) to evaluate risks, benefits, opportunities, and costs associated with reconstruction of Targhee Pass (U.S. 20 between its junction with Idaho 87 and the Montana state line). Targhee Pass is a portion of the U.S. 20 corridor that has been evaluated as part of the U.S. 20 Safety Corridor Study.
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Purpose and Need

The purpose of the proposed project is to improve driver safety, traffic flow, and roadway structural integrity of US 20 between the Junction of SH 87 and the Montana state line, also known as Targhee Pass.

Needs for the Proposed Project

The need for this project is partially documented in the US 20 Corridor Plan (Plan) that was approved by the Idaho Transportation Board in 2006 and by more recent technical reviews by transportation subject matter experts including traffic, geo-technical, wildlife, and pavement maintenance engineers. Specific needs supporting improvement to this portion of US 20 are:

Driver Safety

  • The Plan identified the need to improve safety of US 20 by adding passing lanes and other intersection improvements.
  • The existing road shoulders do not meet current design standards nor do they meet the recommendations of the Plan. Certain types of vehicle crashes will be reduced by widening the road shoulders.
  • Current traffic volumes warrant modification of the traffic flow at the Big Horn Hills Estates entrances and at the Howard Springs pull-out.
  • Forest canopy shading causes road icing in certain locations reducing safety.
  • Blowing and drifting snow contributes to vehicle crashes in the winter.
  • Wildlife-vehicle collisions (WVC) contributed to 10 percent of recorded vehicle crashes on US 20 between Chester and the Montana state line from 2009 to 2014 (Kittleson, 2014).

Traffic Flow

  • Traffic flow is hindered at times by congestion and slower moving vehicles climbing Targhee Pass. Construction of a climbing lane is warranted.
  • The Plan identified the need to improve traffic flow on US 20 by adding passing lanes and intersection improvements.

Roadway Structure

  • Roadway pavement and foundation age exceed the expected life cycle of 40 years. Poor drainage creates soft spots and allows frost heaving of the road during the winter. Recent investigations show the aged road foundation is not suitable for long term pavement stability.


Other important community issues to be evaluated in the Environmental Assessment include:

  • Improving safety for pedestrians and bicycles within the project area.
  • Enhancing wildlife movement across US 20 within the project area. Wildlife movement across US 20 is a safety issue for both drivers and wildlife and can impede migratory, dispersal, and daily movements of wildlife (Clevenger and Huijser, 2011).

Click here to download a one-page study information sheet .