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Delivering a first-rate transportation system for Seattle Scott Kubly, Director







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Curb Bulbs/Curb Extensions/Bulb-outs/Neckdowns

As defined in the Right-of-Way Improvements Manual (ROWIM), curb bulb is a radial extension of a sidewalk at a corner or mid-block location used to shorten the crossing distance for pedestrians, provide access to transit, and expand the landscape/furniture and/or walkable zone. Curb bulbs are a technique used to promote traffic calming.

In addition, the Pedestrian and Bicycle Information Center (PBIC) provides the following description of curb bulbs, also referred to as curb extensions: [They] extend the sidewalk or curb line out into the parking lane, which reduces the effective street width. Curb extensions significantly improve pedestrian crossings by reducing the pedestrian crossing distance, visually and physically narrowing the roadway, improving the ability of pedestrians and motorists to see each other, and reducing the time that pedestrians are in the street.

According to PBIC, the purpose of curb bulbs/curb extensions is to improve safety for pedestrians and motorists at intersections, increase visibility and reduce speed of turning vehicles, encourage pedestrians to cross at designated locations, prevent motor vehicles from parking at corners, and shorten crossing distance and reduce pedestrian exposure. Particularly for pedestrians with impaired mobility, a curb bulb can offer an opportunity to cross a roadway quickly, safely and efficiently.

Curb extensions placed at an intersection also prevent motorists from parking in or too close to a crosswalk or from blocking a curb ramp or crosswalk. Motor vehicles parked too close to corners can present a threat to pedestrian safety when they block sightlines, obscure visibility of pedestrians and other vehicles, and make turning particularly difficult for emergency vehicles and trucks. Curb extensions also provide an excellent place to locate traffic signs, which will be more visible since they cannot be easily blocked by parked cars. The restricted street width provides a visual cue to motorists, encouraging them to travel more slowly at intersections or midblock locations with curb extensions. Turning speeds at intersections can be reduced with curb extensions, making curb radii as tight as is practicable. Curb extensions also provide additional space for curb ramps and for level sidewalks where existing space is limited.

Curb extensions are only appropriate where there is an on-street parking lane. Curb extensions should not extend into travel lanes, bicycle lanes, or shoulders. The turning needs of larger vehicles, such as school buses, must be considered in curb extension design.

Curb Bulb

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