Roadway Width

On this page:

• Design Criteria
• Design Considerations

The term “roadway” refers to the area of the street right-of-way used for vehicular travel, including cars, trucks, bicycles and transit. The roadway may also include a number of additional uses such as on-street parking, curbed structures such as medians and crossing islands, and utility access points.

Standard Plan and Specification References

Standard Plan 400: Half Section Grading
Standard Plan 401: Residential Pavement Sections
Standard Plan 402: Commercial and Arterial Pavement Sections
Standard Plan 405: Types of Joints for Concrete Pavement
Standard Plan 410: Type 410 Curb
Standard Plan 411: Curb Joints and Dowels

Design Criteria

Roadway width on streets with curbs: Where there is a curb, the roadway width is the curb face to curb face width of the street.

Minimum pavement width —non-arterial streets: The minimum right-of-way width varies per Land Use Zone per the Seattle Municipal Code 23.53.015.

When street improvements are required, project applicants shall construct the half street on their side of the-right-of way, plus ensure that a minimum of one – twelve foot paved travel lane and 5 feet of graded shoulder exist on the other side of the centerline. When new pavement is required for this travel lane it shall include a 2-feet wide thickened edge for drainage.If a project is on a block which is split into more than one land use zone category, the zone category with the most frontage determines the minimum width required. If the land use categories have equal frontage, the one with the greater requirement shall be used to determine the minimum street width.

Minimum pavement width — arterial streets: Minimum pavement width requirements for arterial streets are consistent with the minimum design standards given in the Washington State “City and County Design Standards.” 

Projects with Reduced Requirements: Projects with reduced requirements must provide the minimum roadway width specified in the land use code, plus a thickened edge if necessary to control drainage. Projects with reduced requirements in industrial zones should provide the minimum roadway width to meet Fire Code requirements in Appendix ‘D’. 

Lane width for arterials: The following standard arterial lane widths are used in the design of arterials:

Lane Type	Standard Arterial Lane Width	Standard Non-arterial Lane Width
Flex zone	8 feet minimum 9 feet recommended for Industrial Access street types and flex zones adjacent to street car lines	7 feet
Through travel lane	10 feet on one-way streets that are not on the Freight network or on a Transit route 11 feet for Urban Center Connector or Industrial Access street types	10 feet (except Neighborhood Yield street type)
Travel lane adjacent to oncoming traffic	11 feet	10 feet
Travel lane adjacent to curb	11 feet 12 feet on Industrial Access street type	10 feet (except Neighborhood Yield street type)
Bus only lane	11 feet	N/A
Turn only lane	10 feet 11 feet recommended on Industrial Access street type and on HCT and/or FTN transit routes	10 feet

Road taper and transitions: For permanent roadway tapers, the standard taper length for the narrowing from two lanes to one lane or offsetting of a lane is:

• L= WS² /60, where L is the length of taper in feet, W is the lane offset in feet, and S is the design speed in miles per hour.
• On non-arterial streets, temporary asphalt tapers are allowed for narrowing a single lane when additional street improvements are anticipated in the future.
• On non-arterial streets, the standard taper for temporary tapers is 25:1.

Design Considerations

Location of roadway within the right-of-way: The roadway is typically centered in the right-of-way, but may be offset due to topography, existing utilities, or limited right-of-way width.

Considerations for developing minimum roadway dimensions should be made within the context of how the entire right-of-way will be developed over time. This includes public safety; available right-of-way; land use and zoning, current and projected roadway capacity; pedestrian facilities, landscaping (including natural drainage where allowable), bicycle facilities, freight and transit needs, and other intended uses of the public realm.