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Chapter 4
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Design Criteria
4.8 Intersections

The design criteria in this section address elements that are typically located in an intersection. Intersections are a challenge to design and operate because they are the location where traffic turns, pedestrians cross the street, bicycles must navigate vehicle turning movements, the roadway may expand to accommodate more capacity, and medians may become turn pockets.

4.8.1 Links to Standard Plans and Specifications

Standard Plan 422a: Curb Ramp Details
Standard Plan 422b: Curb Ramp Details
Standard Plan 422c:  Angled Curb Ramp Details

4.8.2 Design Criteria

Curb ramps: Curb ramps are located at intersections and other legal crossings to facilitate wheelchair and pedestrian street crossings.  All alteration or new construction projects must follow current ADA requirements.  When a new ramp is installed on one side of the street, per State Law (RCW 35.68.075) an ADA compliant companion ramp shall be installed on the opposite side of the street. If project impacts the legal crossing path, curb ramp or the landing then curb ramps must either be retrofitted to comply current ADA requirements, or new ramps must be constructed that meet the current standard.  Refer to the PORR for thresholds that require ADA upgrades in the curb return area.

Utility location in intersections: Gratings, access covers, and other appurtenances shall not be located on curb ramps, landings, blended transitions, and gutters within the pedestrian access route. 

Curb ramp locations: Curb ramps are permitted only at legal crosswalk locations, at intersections, and at approved marked crosswalk locations. Legal crosswalks at intersections are defined by projections of the curb and back of sidewalk lines right-of-way lines across the street or by a line 10 feet behind the face of the curb or roadway edge when there is no sidewalk. Curb ramps at any other location in the public right-of-way are subject to the approval of the Director of Transportation.

Curb radius : In general, standard curb radius for street intersections are as follows:

When Vehicular Turn is Illegal

10 feet

Arterial to Residential Access

20 feet

Residential Access to Residential Access

20 feet

Arterial to Arterial

25 feet

Arterial to Commercial Access

25 feet

Commercial Access to Commercial Access

25 feet

High Volume Truck and/or Bus Turns

30 feet

SDOT evaluates curb radii based on the type and volume of activity at the intersection. In all cases, with the exception of a location where a vehicle turn is illegal, SDOT will evaluate the curb radii based on a Single Unit (SU) vehicle with a 42 foot turning radius. An applicant may propose tighter curb radii, and will need to provide supporting documentation and have the request approved by the City Traffic Engineer. Refer to Chapter 4.8.3 Design Considerations - Intersections for more information about curb radii.

Emergency vehicle signal priority: Signal priority for emergency vehicles will be included at all new signal installations, and is typically included for traffic signals that are being modified or updated.

4.8.3: Design Considerations

Layout and grading of intersections: The layout and grading of an intersection must be accomplished so that water flows and the intersection is safe and accessible by pedestrians, meets ADA requirements, and is safe for bicycle use.

Curb ramp considerations: Curb ramps should be placed to align with the adjacent crosswalk. Issues to consider include location and placement of utility poles, hand holes, vaults, inlets, catch basins and signal controller equipment.

Curb radii considerations: Curb radii should be designed based on the location and use of the intersection location and should balance the need to accommodate safe large vehicle movements with pedestrian safety:

  • Tighter turn radii are appropriate at intersections that have high volumes of pedestrian and cyclist crossings to support adjacent land uses. These include Main Streets , Mixed Use Streets, Local Connector Streets, and at intersections in Urban Centers and Villages.
  • Wider turn radii are typically required at intersections that experience frequent, high volumes of truck and transit vehicle turns. These include Regional Connectors, Major Truck Streets and streets that are part of the Transit classifications. In these locations, curb radii will be evaluated based on the following standard design vehicle: Single Unit (SU) with a 42’ turning radius. If for some reason, SDOT would anticipate a larger vehicle used in a site, a radius evaluation based on this larger vehicle would be required. Examples of typical turning templates would include a SU, WB-40, WB-50, WB-60 and WB-62.
  • In locations where there is on-street parking in the receiving lane, consideration of tighter curb radii may be appropriate and still allow for safe larger vehicle movements.

Transit signal priority: Traffic signals may be timed to respond to certain conditions at intersections. Transit Signal Priority, transit signal queue jump and other related treatments may be desirable along Regional Connector, Commercial Connector and Industrial Access, Main Street and Local Connector street types, with high transit ridership.

Pedestrian signal priority: Traffic signals at intersections along Local Connector, Mixed Use, Main Street and Green Street street types should consider enhancing pedestrian crossings including pedestrian walk phases on all legs of the intersection, countdown and/or audible signals where appropriate and elimination of pedestrian push buttons.
continue to section 4.9»   
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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 Cul–de–sacs 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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