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Arterial Classification System

Arterial functional classification is the organization of a road system into groups according to the “function” each road serves or is intended to serve. The main functions for any road are 1) to provide mobility for users and, 2) to provide access to adjacent land uses. The degree to which the road serves movement of traffic or access to adjacent land uses is the basis for its functional classification.

Arterial functional classification is used in transportation planning, roadway design and allocation of road improvement funds. In unincorporated King County, there are three types of arterial roadways:

  • Principal arterials—Provide for movement across and between large subareas of an urban region and serves predominantly "through traffic" with minimum direct service to abutting land uses.

  • Minor arterials—Provide for movement within the larger subareas bound by principal arterials. A minor arterial may also serve "through traffic" but provides more direct access to abutting land uses than does a principal arterial.

  • Collector arterials—Provide for movement within smaller areas which are often definable neighborhoods, and which may be bound by arterials with higher classifications. Collectors serve very little "through traffic" and serve a high proportion of local traffic requiring direct access to abutting properties. Collector arterials provide the link between local neighborhood streets (i.e. non-arterials) and larger arterials.

The majority of the roads in the King County road system are not designated as arterials and function as local neighborhood streets.

The Arterial Functional Classification Map is a component of the King County Comprehensive Plan (Technical Appendix C – Transportation), and is reviewed periodically for system changes. Changes to the arterial classification are handled through the Comprehensive Plan review and adoption process. The Comprehensive Plan is normally reviewed annually, with a major update scheduled every four years. Citizens can request changes to the Comprehensive Plan through the formal "Comprehensive Plan Amendment Docketing process".

Concurrency Management Program

Current status

The Transportation Concurrency program has been updated effective March 18, 2017. This update includes: 1) A more streamlined and efficient process for data collection, 2) Updates to travel shed boundaries reflecting incorporations and annexations as well as separation of urban and rural areas so they can be tested to their adopted level of service standards, and 3) Deletion of regionally-significant state routes from Concurrency testing. The updated ordinance, annual report, and travel shed boundary map and test results map are listed below.

The current adopted Concurrency regulations and level of service standards can be found in Title 14, Chapter 14.70 of the King County Code.

About transportation concurrency

Concurrency is a land use regulatory framework under the Washington State Growth Management Act (GMA), passed in 1990. It is used by local governments to ensure that sufficient public facilities are in place for any new land use development. It also supports the overarching goals of the GMA: Focusing growth in urban communities, reduction of sprawl, and protection of natural and agricultural resource lands.

King County first implemented the Transportation Concurrency Management program by adopting its first Comprehensive Plan following the GMA in 1994, with the program becoming effective in 1995. Transportation concurrency policies can be found in Chapter 8 ("Transportation") of the current adopted King County Comprehensive Plan.

Key elements of the program are summarized as follows:

  • Travel time data is collected by corridor segments on County principal and minor arterials
  • Concurrency testing is by geographic “travel sheds” where travel patterns share common characteristics
  • Concurrency testing evaluates travel time by roadway mileage within each travel shed
  • The concurrency map is reviewed and updated every two years or when directed by the King County Council
  • Rural Mobility Areas are designated rural areas (the towns of Vashon, Snoqualmie Pass and Fall City) that support a greater variety of transportation mode choices than typically found in most rural areas. These areas have level of service standards which support and encourage people to use alternative modes of transportation
  • 85 percent of travel shed arterial mileage must meet concurrency standards for the travel shed to pass the concurrency test
  • Residential and nonresidential proposals are tested by the same concurrency map
  • Concurrency is based on pass or fail of travel shed status as shown on the concurrency map;
  • Clear identification is made of failing corridor routes that are the primary reason a travel shed is out of compliance

Frequently-asked questions

Check your property's transportation concurrency status using the King County Parcel Viewer. Input your property's address or 10-digit parcel identifier. After the application shows your property information, select "Districts Report" located on the lower left-hand side of the page. On the "Districts Report" refer to the row titled "Transportation Concurrency Management" for your "Pass" or "Fail" determination. Technicians at the Department of Permitting and Environmental Review (DPER) handling your application will check this on your pre-application form.
Should your property be identified as failing concurrency, check with a DPER staff member to see if your property or development proposal qualifies under the category “Minor Developments and Public and Educational Facilities,” which may allow you to proceed with your application (see King County Code 14.70.285). If the proposal does not meet these criteria, you cannot proceed with your development proposal until the property is identified as passing concurrency.

Mitigation Payment System

The King County Council withdrew the Mitigation Payment System program, effective December 17, 2016. This program required new development in the unincorporated area of the county to pay fees for traffic impacts to the county road network. These fees were used to help offset the costs of certain county road improvement projects that support growth. Mitigation Payment System fees were just one aspect of the transportation mitigation requirements for new development. Decades of annexations and incorporations have left unincorporated King County with primarily rural areas and low growth rates. As a result, Mitigation Payment System fee revenues have declined steadily and no longer justify the expenditure of resources to administer the program. Impacts of new development on the transportation system can still be mitigated through the State Environmental Policy Act process and the county’s intersection standards requirements.

Transportation Needs Report (TNR)

About the TNR

The TNR is a list of unincorporated King  County improvements recommended by the King County Road Services Division and approved by the Metropolitan King County Council to serve unincorporated King County's transportation needs, projected to the year 2031. 

The TNR is a component of the King County Comprehensive Plan and fulfills state requirements as specified in the Washington State Growth Management Act.

Current status

The latest version of the TNR was adopted by the King County Council on December 5, 2016 as part of the 2016 King County Comprehensive Plan update. The following is the final adopted version of the TNR.

Please note, if you are using Internet Explorer, you may need to download the PDF files provided in the links below in order to view them properly. You may do so by right-clicking on a link below and selecting "save target as" to save the file to your computer for viewing.

2016 Transportation Needs Report

Subarea boundary maps

Differences between the TNR and Capital Improvement Program (CIP)

The CIP differs from the TNR in that the CIP contains projects which have some degree of funding associated with them. The CIP is updated annually, while the vast majority of TNR projects are currently unfunded. The TNR operates in the long-term time frame of the Comprehensive Plan, while the CIP covers the next six years of expected funding. For the current list of funded transportation projects, please refer to the CIP program.

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