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I-405 Corridor Program, Seattle, WA


Transportation Corridor Planning and Tier 1 Environmental Impact Statement (EIS.

Implementing Agency

Washington State DOT (WSDOT)


From 1999-2002, WSDOT and several other partners developed a master plan for the heavily-congested corridor along Interstate 405 in east suburban Seattle. The corridor team also produced a Tier 1 Environmental Impact Statement (EIS), a streamlined planning document that addresses large-scale issues, providing guidance and background for incorporation into the environmental assessments of the Corridor Plan's 300 proposed component projects.

Responding to land use policies along the Corridor calling for concentrated growth in Urban Growth Area cores, "mart growth" land use strategies account for approximately $95 million of the nearly $11 billion in estimated costs for the 300 projects identified by the preferred alternative in the Corridor Plan and EIS.

Link to Livability

The Washington State DOT I-405 Corridor plan uses the corridor to link several urban centers. The plan directs transportation investment toward existing urban centers, and outlines a strategy for transit oriented development with bus rapid transit service, which will help reduce congestion and improve accessibility in the area. By conducting a Tier 1 EIS for the entire corridor, the plan is able to develop comprehensive approaches to environmental mitigation by conducting the environmental assessments on a broader level.

Context and Background

Originally intended as a bypass route, the 30-mile corridor of Interstate 405 in east suburban Seattle is Washington State's second most heavily traveled expressway. High growth in population, employment, and traffic congestion characterize the largely suburban region that surrounds the corridor. Based on a mutual goal to improve mobility in the corridor, WSDOT, the State legislature, residents, businesses, and local leaders cooperated in the development of a plan to address transportation needs in the 1-405 Corridor. The plan incorporates information from local land use plans and metropolitan area policies related to land, and also includes environmental documentation for each of the proposed projects.

Detailed Description

In 1999, WSDOT initiated the I-405 Corridor Program. Backed by strong financial support from the state legislature and a desire among local communities, leaders, and interest groups to address regional mobility concerns, the program completed both a corridor-wide Tier 1 EIS and a long-term master plan. In the EIS, WSDOT considered several transportation alternatives including a transit- and management-focused alternative, a general capacity (freeway expansion) alternative, and two "mixed mode" alternatives combining infrastructure expansion with management strategies and transit. The Corridor Plan focuses on the recommended "mixed mode" alternative and describes its components, a timeline, and funding needs over the next 20-30 years.

The plan featured two new expressway lanes in each direction, 15 access and exit points for HOV lanes, ten bus rapid transit stations, 5,000 new park-and-ride parking spaces spread over about 20 facilities, nine transit centers, expanded transit and vanpool service, development of activity centers through land use changes, and a comprehensive transportation demand management program. Figure 1 displays the I-405 Corridor Plan area map.

I 405 master plan map

Figure 1 I405-Corridor Plan Map | I-405 Corridor Plan, WSDOT 1999

The I-405 Corridor Plan envisions a multi-modal program of improvements, including land use strategies and community amenities as well as expansion of general purpose freeway capacity, high occupancy vehicle (HOV) lanes, vanpools, and transit, as the most appropriate and effective approach to addressing mobility problems.

Urban Growth Areas and Urban Centers

Underlying the plan is a strategy of investment in transportation facility expansion and improvement within the Seattle region's designated Urban Growth areas..

All of the investments are focused within the designated growth area surrounding east Seattle. The plan offers the region's leaders and residents an outline for future transportation strategies. This plan will also help communities comply with the State of Washington's growth management policies which call for concurrency of adequate transportation facilities for all new development which will increase trips.

The Growth Management Act designates urban centers, establishes density targets, and recommends minimum levels of service on key state highways. I-405 is located within the Seattle-area growth boundary. This map illustrates the urban centers i nthe Central Puget Sound region - largely centered around I-5 and I-405.

Figure 2 Urban Centers | I-405 Corridor Plans, WSDOT 1999

The plan also follows local land use policies to promote growth and higher densities in Urban Centers (shown in Figure 2) within the Urban Growth Areas. This element is critical, as the Corridor Plan does not simply react to increased population projections along Route 405. Rather, it focuses on creating linkages between Urban Centers (the downtown areas of selected municipalities), which will help drive infill development, and discourage development and the extension of urban services and infrastructure in rural areas.

Tier 1 Environmental Impact Statement

As a product of the planning process, a Tier 1 EIS was completed to address environmental issues of broad concern throughout the corridor. This task allowed planners to address the environmental impacts of the corridor as a whole, and also serves as a first step for developing individual impact statements for individual projects within the plan. The Corridor level Tier 1 EIS also helps planners develop optimal mitigation and protection for the area's natural resources, namely the rivers and salmon habitats.

Using the Tier 1 EIS as a guide to identify issues that have already been discussed and debated by the community, the assessments can be carried out much more quickly, saving money and advancing the objectives of the plan.

Corridor Executive Committee

One of the program's early steps was designation of a corridor executive committee, consisting of local, State and Federal representatives, to provide vision, policy, and oversight in the development of the I-405 Corridor Program. Through monthly meetings open to the public, the committee ensures that public involvement remains a critical input into the decision-making process. The committee was instrumental in holding together the coalition of stakeholders within the corridor throughout the development of the plan and during the beginning of its implementation. The committee is involved in the beginning planning stages for component projects within local jurisdictions.

Plan Implementation and Funding

Implementation of the projects included in the Corridor Plan faces considerable financial constraints, and leaders recognized that they would not receive the estimated $11 billion in funding needed for full implementation. While the state legislature levied a five-cent gas tax and committed $485 million to I-405, these actions raised less money for project implementation than originally planned through a higher proposed gas tax. The legislature also authorized formation of the Regional Transportation Improvement District (RTID) to raise funds through taxes and fees within the three-county Seattle area and apply funds to projects within the district's boundaries. I-405 supporters anticipate much of the funding for the remainder of the projects will come from the RTID as well as other local, state, and federal sources. Innovative alternatives such as regionally-based congestion tolling, considered as part of the EIS, will also complement traditional sources of transportation financing.

As with a long-range transportation plan, corridor stakeholders realized that the conditions and priorities within the corridor would evolve over time. Implementing agencies and member jurisdictions pragmatically view the plan as a strategy for long-term, future investment. As funds for the corridor become available, leaders will apply them according to the goals and priorities as described by the plan.

Application Examples

Cities of Renton and Kirkland

One example of a city undertaking capital improvements to emphasize its Urban Center is the City of Renton. The City is partnering with transit agencies and private developers to construct mixed-use developments, which are transit supportive land in the City's designated Urban Center.

The City of Kirkland is also following up on its plans to develop transit and mixed-use activity centers, through tasks that are indicative of similar work in other cities.

Project managers emphasize that concentration of new transportation facilities, infrastructure improvements, and expanded transit service is within the Urban Growth Area for the Seattle region, as designated under state law. As the populations of towns such as Kirkland and others continue to grow, facilitated in part by the concentration of transportation improvements within the growth boundary, towns will have strong incentives to increase densities. The Corridor Plan recognizes the necessity of increasing the densities of the Urban Centers and, as a result, emphasizes the value to communities of encouraging transit villages, mixed-use developments, and higher density developments. Planned HOV and HOT lanes, and bus rapid transit will create further incentives for increased densities in Urban Centers and along the Corridor.

Increased options for transportation and mixed use development are two key components of livable communities. This Corridor plan supports both concepts through strategic planning and coordination with local communities.

Nickel Projects

The first funded components of the Corridor Plan are three distinct segments of general purpose lane additions to I-405, two of which have been completed and one which is under construction, as of July 2010. Known as the "Nickel Projects," due to their funding source from the five-cent gas tax-to expand highway capacity, these three improvements underwent individual environmental assessments, which involved community meetings to identify areas of particular concern, using the Tier 1 EIS for background. Because the Tier 1 EIS considered alternatives for the entire program of improvements for I-405, the component projects' Environmental Assessments focus on more project-specific issues such as culvert crossings, final roadway alignments, sound wall design, and traffic control plans for construction.

Sound Transit Access Ramp

The comprehensive nature of the Corridor Plan allows for time savings and more sensible timing and design of projects. For example, Sound Transit (the transportation agency for Central Puget Sound in Washington) is building a direct access overhead ramp to the Interstate to serve express buses. Because the project team can anticipate future expansion of the Interstate, as called for in the Corridor Plan, the design of the bus ramp will accommodate the future geometric needs of the expanded travel lanes beneath.

Lessons Learned

Maintain a long-term vision.

It is important to keep the long-term vision for the corridor as a whole, despite the reluctance from localities interested in advancing individual projects. A bigger picture, long-term vision allows the project to address key issues and needs comprehensively.

Conduct proper public outreach and education, especially highlighting the corridor as an interdependent system.

Because funding and resources are constrained, as funding becomes available, higher priority projects can be advanced to make best use of available resources. With careful outreach and communication, local communities will understand that they are part of a larger system.

Transportation infrastructure is an important element to encourage infill development.

A major goal of the plan was to support Washington State's land use policies to encourage denser developments at Urban Centers. Concentrating transportation infrastructure in urban cores can drive compact, mixed use and infill development.

Multi-modal options and intelligent transportation systems help to increase mobility and accessibility.

Employing multi-modal options such as bicycle and pedestrian networks, and transit systems can reduce congestion while improving accessibility to all people, including those who cannot or choose not to drive. Intelligent transportation systems can improve mobility while preserving existing infrastructure assets, reducing the need for developing new roads.

Bicycle and pedestrian links between neighborhoods support livable communities.

The plan addresses intersection safety improvements and other areas of bicycle and pedestrian connections between neighborhoods, demonstrating a key component of livability - safe routes for all travelers, including bicyclists and walkers.

Conduct early planning, especially in regards to environmental permitting.

Rather than conducting environmental analysis at a project level, conducting environmental assessments at a regional or corridor scale early in the process can expedite the later process of individual project permitting because much initial analysis will already be complete. An environmental analysis at the corridor level can also help provide more appropriate mitigation solutions for the larger area.

For Further Information


Kim Henry
Project Director

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Updated: 10/20/2015
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