Curb extension

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A curb extension marked by darkened tarmac and black posts.

A curb extension (or also kerb extension, bulb-out, kerb build-out, nib, elephant ear, curb bulge and blister) is a traffic calming measure, intended to slow the speed of traffic and increase driver awareness, particularly in built-up and residential neighbourhoods. They also allow pedestrians and vehicle drivers to see each other when vehicles parked in a parking lane would otherwise block visibility.

A curb extension comprises an angled narrowing of the roadway and a widening of the sidewalk (pavement in UK usage). This is often accompanied by an area of enhanced restrictions (such as a "no stopping" or "no parking" zone) and the appropriate visual reinforcement. This is achieved using painted road markings (e.g. lines, coloured areas, or chevrons), barriers, bollards, or the addition of pavement or street furniture (e.g. planters, lamp standards, or benches).

Curb extensions are often used in combination with other traffic calming measures such as chicanes, speed bumps, or rumble strips, and are frequently sited to "guard" pedestrian crossings. In these cases the "squeeze" effect of the narrowed roadway shortens the exposed distance pedestrians must walk.

Curb extensions can pose a hazard to cyclists, as they force cyclists from their position at the road side (or in a roadside bike lane) into the narrowed gap. Consequently, many curb extensions are built with the bike lane passing through (making the extension an island, separated from the main sidewalk by a narrow bike lane).

Broadway at 33rd Street, Manhattan.

Curb extensions are also used in a number of special circumstances:

To provide additional horizontal space to allow retrofitting of existing sidewalks with ramps, where the sidewalk would otherwise be too narrow.
To provide additional visibility and protection for pedestrians (particularly children) when leaving premises. The curb extension may contain a pedestrian barrier, preventing pedestrians from running straight from the premises over the road.
In combination with a controlled urban parking scheme, where parking spaces are shielded from oncoming traffic by the extended sidewalk element.
At a four-way (signalled) intersection, to slow and calm traffic, particularly fast traffic turning from a major to a minor road.
To protect passengers embarking and particularly disembarking from trams, buses, and level-grade urban light rail systems, particularly when retrofitting existing streets.

Curb extensions are also used when retrofitting existing streets to accommodate congestion charging schemes - hitherto wide (sometimes multi-lane) roads are deliberately narrowed to ensure that the charging equipment can see passing vehicles, and the charging equipment (and often bollards or other barrier devices) are placed in the expanded sidewalk area (to prevent drivers from circumventing the charging system's cameras and detectors).

To facilitate street sweeping, the internal and external curvatures of the extended curb section are moderately graduated.