Garrity Interchange Design

Document Type


Publication Date

April 2010

Faculty Sponsor

Dr. George Murgel and Dr. Molly Gribb


The Garrity Boulevard interchange in Nampa, Idaho was built in the mid 1960’s as part of the original build-out of the interstate system funded by the Federal-Aid Highway Act of 1956, (also known as the National Interstate and Defense Highways Act). The interchange accesses I-84 (west), originally called I-80N, and was built during a period when Nampa was experiencing steady population growth, and residential and commercial construction such as the Karcher Mall (the first indoor mall in the Treasure Valley). Since construction of the interchange Nampa’s population has ballooned 382% (1970-2007). The American Association of State Highway Transportation Officials (AASHTO) describes quality of traffic flow on a roadway with a Level of Service (LOS) letter designation, A through F. LOS D is defined as ‘Approaching Unstable Flow’, which is typical of an urban arterial during busy travel hours. A LOS of E or F indicates ‘Unstable Flow’ or ‘Breakdown Flow’, which can be an indicator that a roadway has reached its capacity, resulting in traffic jams and heavy congestion during peak hours. The two existing I-84 two-lane bridges are nearing the end of their design lives and currently meet traffic demands with a LOS of D. Widening of the bridges and ramps is required to meet future traffic demands and complement I-84 widening projects adjacent to the East and West. This senior design project is the evaluation and redesign of the Garrity Boulevard interchange and the I-84 overpass to address future traffic demands. Our design will include: 1) Analysis of geotechnical reports to determine the properties of the soils and road materials currently in place, 2) Design and layout of the additional lanes on Garrity Blvd and I-84, 3) Design and construction of a new bridge, 4) Redesign and improvement of the intersections, 5) Redesign of the transitions between the on and off ramps and the interstate, and 6) Design of a storm water runoff management system. The new design and layout will allow the new I-84 bridge to support four lanes of traffic in each direction and will add bicycle lanes and sidewalks to Garrity Boulevard. Reconstruction of the interchange will provide the required capacity to support future traffic flows and increased safety to the traveling public. The redesign of the intersections will improve traffic control systems that are currently in place and will include the removal of existing pier structures currently slowing traffic flow on Garrity Blvd.

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