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Chapter 4
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Design Criteria
4.6 Roadway Width

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.

4.6.1 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

4.6.2 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.

Typical Design Cross Sections are provided in the figures below.

Figure 4-2:  Crushed Rock Improvement
Figure 4-3:  Crushed Rock Improvement Industrial Zones
Figure 4-4:  Crushed Rock Edge Detail
Figure 4-5:  Asphalt Concrete Pavement:  New Pavement For Streets without Existing Hard Surface
Figure 4-6:  Asphalt Concrete Pavement:  New Pavement For Streets without Existing Hard Surface in Industrial Zones
Figure 4-7:  Asphalt Thickened Edge Detail
Figure 4-8:  Asphalt Concrete Pavement:  Pavement Widening For Streets Existing Hard Surface Streets without Sidewalk
Figure 4-9:  Pavement Widening for Existing Hard Surface Streets
Figure 4-10:  Curb and Sidewalk Improvement: New Pavement For Streets Without Existing Hard Surfaces
Figure 4-11:  New Concrete Sidewalk with Existing Curb
Figure 4-12:  Full Improvements for Newly Dedicated Streets
Figure 4-13:  Alley Improvement

Minimum pavement width —non-arterial streets: The minimum roadway width varies per Land Use Zone per the table below.

Zone

Standard Roadway Width

Low Density Residential

SF, LDT, L1, NC1

25 Feet

Mid-Density Residential and Mixed Commercial / Residential

L2, L3, L4, NC2-30, NC2-40, NC2-65

32 Feet

High Density Residential, Mixed Commercial / Residential, and Industrial

NC3, MR, HR

36 Feet

C1, C2, IB, IC, IG1, IG2

40 Feet

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.

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 (see figures above).

Minimum pavement width — arterial streets: Minimum pavement width requirements for arterial streets must meet the minimum design standards given in the Washington State “City and County Design Standards.” Refer to Appendix A: Arterial List for existing and required right of way and roadway widths for arterials.

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.  28 feet is recommended by SDOT.

Lane width for arterials: The following standard arterial lane widths are used in the design of arterials and are compliant with the City and County Design Standards:

Lane Type

Standard Lane Width

Parking lane

8 feet

Parking lane on bus route

10 feet

Through traffic lane

11 feet

Curb lane

12 feet

Bus only lane

12 feet

Turn only lane

12 feet

Curb lane (vehicle/bicycle)

14 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.
4.6.3 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.

Considerations for establishing lane widths should be influenced by street type designations, street classifications, and the need to have a reasonable balance among competing uses in the right-of-way. They are as follows:

  • Lane widths need to support large vehicle movements such as trucks and transit.  Wider lanes should be considered on Regional Connector and Industrial Access street types.
  • Wide center turn lanes should be considered on streets that have high volumes of truck turning movements including intersections with one or more legs that are designated as Major Truck Streets or Industrial Access streets.
  • Wide curb lanes should be considered on streets that have high volumes of transit service.
  • On Commercial Connector streets, lane width should support travel demand.  Areas where high pedestrian activity is occurring, consider options that keep lanes as narrow as possible. On streets with high levels of transit service or that accommodate high volumes of trucks and transit vehicles, consider options that have wider lane widths.
  • Narrow lanes support slower vehicle speeds, and minimize overall roadway width which supports pedestrian activity. Narrower lanes can be challenging for larger vehicles to navigate, especially on roadways that carry high volumes of trucks and transit vehicles. Narrower lanes should be considered along Local Connectors, Mixed Use, Main Street and Green Street street types. For Green Streets and Neighborhood Green Streets, keep lane widths narrow and limit the number of lanes to minimize the crossing distance for pedestrians as much as possible.
Lane widths on designated bicycle routes shall be evaluated based on the lane width requirements given in the City and County Design Standards and WSDOT Design Manual.
continue to section 4.7»   
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Detailed Table of Contents
Chapter 4
Design Criteria
4.1 Introduction
4.2 Street Classifications and Street Types
4.3 Design Criteria General Notes
4.4 Grading
4.5 Design Cross Section
4.6 Roadway Width
4.7

Roadway Pavement

4.8 Intersections
4.9 Driveways
4.10 Curbs
4.11 Sidewalks
4.12 Crosswalks
4.13 Bicycle Facilities
4.14 Street Trees and Landscape Architectural Standards
4.15 Introduction to Utilities Design Criteria
4.16 Street Lighting
4.17 Street Drainage, Storm Drains and Sewers
4.18 Water Mains
4.19 Fire Protection
4.20 Seattle City Light
4.21 Clearances
4.22 Structures in the Right-of-Way
4.23 Culdesacs and Turnarounds
4.24 Traffic Operations
4.25 Transit Zones
4.26 Street Furniture, Public Art and Unique Objects in the Public Right-of-Way
4.27 Access Easements
4.28 Contact Information
   
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