I-84 Mobility Improvement Study

Why We Need this Study

Continued development, traffic, and growth have strained I-84 during the morning and evening commutes.

• Traffic volumes on I-84 have increased over the last 5 years by an average of almost 10%, with larger increases in the western segments.
• The on-ramps from the interchanges to I-84 experience significant congestion during commute times and highway users often back up onto cross streets.
• The off-ramps have similar congestion issues during commute times, backing traffic up onto I-84.

Study Alternatives

Compare effectiveness of alternatives that would widen I-84 to five lanes in each direction or add an auxiliary lane in each direction between each I-84 interchange.

Investigate intelligent transportation system (ITS) strategies and ramp metering to determine if options will work with the alternatives described above to reduce congestion.

ITS strategies identify real time information on the freeway and share it with travelers and ITD, allowing ITD to adjust signals to better serve traffic. These could include:

• Travel time displays on overhead signs to let travelers know how long it will take to reach their destination.
• Automatic incident detection to let ITD and travelers know of crashes and incidents on the freeway.
• Stopped vehicle detection system to let ITD and travelers know of vehicles stopped on the freeway.
• Traffic signal control systems to better coordinate signals to keep traffic moving.

Ramp meters are traffic signals placed on freeway on-ramps to control how vehicles enter the flow of traffic on the freeway. As seen in the figure on this page, vehicles traveling from the local road onto the ramp line up at the stop line when the signal is red. The vehicles drive and merge into freeway traffic when the signal turns green.

Ramp metering programs across the United States show increases in travel speed on the freeway, reduced congestion and delay on the freeway, and safety benefits allowing smoother merging and reducing collisions. There is often increased congestion and backing up of traffic on the ramp and the local road network.

ITD will not be investigating high-occupancy vehicle (HOV) lanes as that feature does not comply with state law.

Next Steps

ITD will develop a phasing plan showing recommended improvement projects and what order they should occur in. These projects will be considered for inclusion in the annual update to ITD’s Idaho Transportation Investment Program (ITIP) budget document. Once selected, a project must still be designed and receive environmental clearance before starting construction.

The study started January 2025 and will be complete by summer of 2026. Then, ITD will host an online public information meeting presenting results of the study.

As the planning study continues, this document will be updated to incorporate new information and address emerging questions.

---