I-15 US20 Design Summary Report The Transportation System Alternatives (TSA) Study, conducted by DKS Associates for ITD District 6 and the Bonneville Metropolitan Planning Organization (BMPO) in 2010, found that the highway infrastructure in the core area of the Greater Idaho Falls area would not be adequate to accommodate the growth expected in the region by the year 2035. In particular, the aging infrastructure surrounding the interchange of I-15 and US-20 would not be able to carry the volume of traffic forecast for the corridor and significant breakdown and delay would occur if significant improvements are not made.

The purpose of this project has been to explore the present and future operational characteristics of the corridor and to identify potential low or medium cost projects that could extend the life and functions of the corridor alone or in combination with other more expensive long-term projects. 

Ultimate solution of the problems at and near the I-15/US-20 interchange will not be simple of inexpensive.  A combination of strategies were recommended by the TSA Study to meet the future needs of these two roadways cost-effectively and in a manner consistent with the principles for meeting other roadway needs of the metropolitan area. The study recommended the development of an inner strategic arterial loop and outer beltway be designed to provide reasonable alternatives for the I-15 to US-20 movements to the extent possible.  These two beltways would require alternative connections between I-15 and US-20 to the north of the present interchange.

The TSA Study also recommended physical improvements to the existing I-15/US-20 interchange be considered that would provide more throughput at a reasonable level of service. The study recommended a detailed corridor study be initiated to evaluate the options for improving the interchange performance and the performance of the roadways near the interchange. This current effort is in response to that recommendation. The project was designed to examine the need for operation improvements in three time frames – current conditions (2013), mid-term (2020) and long-term (2035).  For the long-term conditions, the project also examined the need for and potential effectiveness of low to medium cost improvements with and without an inner arterial beltway providing an alternative connection between I-15 and US-20 north of the present interchange.

This project focused on the operation of I-15 and US-20 between the I-15/Broadway interchange and the US-20/Science Center Drive interchange in Idaho Falls, and included explicit consideration of the I-15/US-20 and US-20/Fremont Avenue interchanges.  The operation of the corridor was evaluated using the microsimulation software VISSIM for present day and expected future traffic demands.

The project corridor not only serves traffic generated within the Greater Idaho Falls area, it also serves major through routes that carry regional and interstate traffic.  I-15 acts as the main north-south route through eastern Idaho, connecting Montana to Utah.  US-20 carries recreational, agricultural and commute traffic northeast from Idaho Falls toward Yellowstone National Park in Wyoming.

The main problems currently experienced by users of the corridor revolve around the intersection of US-20 and the I-15 ramps.  Current problems experienced by users are delay for users travelling westbound on US-20 to southbound on I-15.  Users taking the northbound I-15 off ramp to US-20 currently cause queuing to the gore point on that off-ramp, indicating that increased traffic volumes will result in queue spilling onto mainline I-15, which is an operational as well as safety problem. Forecast for 2020 and 2035 indicate that the problem becomes exponentially worse with growth in traffic.  Back-up for the intersections will eventually spill back and clog the operation of the interchanges and ramps for Broadway and Fremont Avenue.