The design criteria in this section addresses 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.

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

Curb ramps : two compliant curb ramps with tactile warning strips are required to be installed at each impacted corner and corresponding compliant companion ramps need to be retrofitted or constructed. Curb ramps and companion ramps are required whenever more than 3 lineal feet of curb or 12 square feet of sidewalk is being added, repaired, or replaced at the pedestrian landing area. If a street paving impacts legal crossing path or the landing modified, then curb ramps must either be retrofitted to comply with Standard Plan 422, or new ramps must be constructed that meet the current standard.

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 SDOT Director of Seattle Department of Transportation.

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

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 Section 4.8.3 Design Intersection Considerations 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.

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 and safe for bicycle use.

Utility vault location in intersections : Do not place vaults, covers, castings or drainage grates within the crosswalk, curb ramp or landing area behind or in front of the ramp. In the event that no feasible alternative exists, SDOT will work with the applicant and Seattle City Light to determine the best possible location.

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.

Any curb ramp constructed prior to January 2003 which does not comply with current standards would at least need to be retrofitted according to the design criteria described above.

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.