The SR 18 Traffic Revolution: 5 Critical Changes Washington Commuters Must Know For 2025

The traffic landscape on Washington's State Route 18 (SR 18) has been permanently reshaped, with 2025 marking the culmination of one of the most significant transportation projects in King County history. As of today, December 17, 2025, commuters and freight haulers are navigating a drastically different highway, thanks to the completion of the I-90/SR 18 Interchange Improvements and Widening project. This multi-year effort by the Washington State Department of Transportation (WSDOT) has delivered a brand-new Diverging Diamond Interchange (DDI) and expanded the critical stretch of highway between Interstate 90 and Deep Creek, fundamentally altering travel times and safety on this vital east-west connector.

The days of perpetual congestion and dangerous single-lane stretches over Tiger Mountain are officially over. The project, funded in part by the 2022 Move Ahead Washington transportation package, was specifically designed to address the notorious bottleneck where SR 18 meets I-90 and to improve the accident-prone corridor. Understanding the new traffic patterns, the recently opened two-lane configuration, and the final construction closures is essential for anyone traveling between the South Puget Sound area and the Snoqualmie Valley.

The SR 18 Profile: A Crucial King County Corridor

State Route 18 (SR 18) is far more than just a mountain pass; it is a critical artery for Washington State's economic and commuter life. Its history and route profile underscore why the recent WSDOT improvements were so necessary.

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SR 18 Highway Biography and Key Entities

• Official Designation: State Route 18 (SR 18).
• Length: Approximately 28.41 miles (45.72 km).
• Location: Primarily serves southeastern King County.
• Termini: The highway connects the South Puget Sound area, beginning near Federal Way/Auburn, and terminates at Interstate 90 (I-90) near Snoqualmie.
• Historical Context: SR 18 was established during the 1964 state highway renumbering, succeeding the Auburn–Federal Way branch of Primary State Highway 5 (PSH 5).
• Crucial Role: It is the primary connection between the I-5 corridor (via SR 167) and the I-90 corridor, serving as a vital freight route and a popular alternative to Snoqualmie Pass during winter conditions.
• Notoriety: The section over Tiger Mountain was historically known as one of Washington State’s most dangerous roads due to its steep grades, sharp curves, and single-lane configuration, which led to a high rate of collisions.

The constant growth in the Snoqualmie Valley and the greater Puget Sound region made the old single-lane configuration unsustainable, leading to the massive I-90/SR 18 Interchange Improvements project, which began in 2022.

5 Critical Traffic Changes on SR 18 You Must Know for 2025

The year 2025 brought permanent, structural changes to SR 18 traffic flow. These are not temporary construction detours; they are the new reality for Washington commuters.

1. The New Diverging Diamond Interchange (DDI) at I-90 is Open

The single biggest change is the opening of the new I-90/SR 18 Diverging Diamond Interchange (DDI), which went into full operation in July 2025. This innovative design temporarily shifts traffic to the opposite side of the road at the interchange, which WSDOT implemented to dramatically reduce conflict points from 26 down to 14. For drivers, this means:

• Left turns onto I-90 (both eastbound and westbound) from SR 18 are now free-flowing and do not require waiting for a traffic signal.
• The DDI is designed to handle the growing traffic volumes more efficiently, significantly improving flow onto the Snoqualmie Parkway and I-90.

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2. SR 18 is Now a Four-Lane Highway from I-90 to Deep Creek

The notorious two-mile bottleneck between I-90 and Deep Creek has been eliminated. Following a major full closure in October 2025, eastbound and westbound State Route 18 reopened with two lanes in each direction in this critical stretch. This widening project has been the core focus of the construction since 2023, and its completion is expected to drastically reduce the severe congestion that plagued the corridor during peak hours.

3. Full Closure Over Tiger Mountain for Final Work (August 2025)

To finalize the widening and perform annual maintenance, WSDOT implemented a full four-day closure of SR 18 over Tiger Mountain, specifically between I-90 and Issaquah-Hobart Road, in August 2025. Although the main widening is complete, these final closures were necessary to ensure the structural integrity and long-term safety of the newly expanded highway. Local traffic to SE 104th Street was the only exception during this period.

4. The SR 18/SR 167 Interchange Remains a Focus Area

While the I-90 section received the DDI, the interchange near Federal Way where SR 18 meets SR 167 Northbound continues to be an area WSDOT monitors closely. As recently as December 14, 2025, a ramp closure was reported on the SR 18 eastbound off-ramp to SR 167 Northbound (MP 3) due to water over the roadway. This highlights the ongoing need for drivers to check WSDOT's real-time updates for conditions on the western end of the highway, particularly during heavy rain or adverse weather.

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5. Reduced Collision Risk Due to Infrastructure Improvements

The entire purpose of the I-90/SR 18 Interchange Improvements project was to enhance safety. The new DDI design alone is projected to reduce the opportunity for collisions by 50% by minimizing confusing merge and turn points. Furthermore, the elimination of the single-lane bottleneck over Tiger Mountain significantly reduces the risk of head-on collisions and rear-end accidents that were common in the congested, old configuration. The widening project's goal aligns with WSDOT's commitment to making the historically dangerous route safer for all users.

Navigating the New SR 18: Essential Tips for Commuters and Freight

With the major construction phase largely complete and the new infrastructure in place, the focus shifts to how drivers can best utilize the improved corridor. The new traffic patterns require a slight adjustment in driving habits.

Understanding the Diverging Diamond (DDI)

The DDI at I-90/SR 18 is counter-intuitive but highly effective. Drivers traveling through the DDI should pay close attention to the overhead signage and lane markings. The temporary shift to the left side of the road is brief and designed to maximize efficiency. Do not panic when you see traffic crossing over; this is the intended design to facilitate uninterrupted left turns onto the interstate ramps.

WSDOT Resources and Real-Time Information

The most reliable source for current conditions remains the Washington State Department of Transportation (WSDOT). Commuters should utilize the WSDOT Traffic App and website for real-time updates, especially concerning the western sections of SR 18 near Federal Way and Auburn, where localized issues like water over the roadway can still occur. Subscribing to WSDOT's email updates for the SR 18 corridor is the best way to stay informed about any minor maintenance closures or unexpected incidents.

The Role of Stakeholders and Future Planning

The success of the I-90/SR 18 project was a collaborative effort involving WSDOT, the contracting community, and various stakeholders, including the Executive Advisory Group and the Stakeholder Advisory Group (SAG). While the major widening is complete, communication between these entities remains crucial for future maintenance and potential further improvements to the highway. The project serves as a model for how the state can tackle other high-volume, high-risk corridors in the future.

The "SR 18 traffic" keyword now refers to a highway that is modern, safer, and significantly more efficient. The completion of the DDI and the widening over Tiger Mountain represents a monumental win for Washington commuters and a major step forward in regional transportation infrastructure.