Curb extension

From Wikipedia, the free encyclopedia.

(Redirected from Curb)

Nb. Kerb also has a historic meaning. American TV Comedy Curb Your Enthusiasm is commonly referred to as Curb.

A curb or kerb is the edge where a raised sidewalk, road median, or road shoulder meets an unraised street or other roadway. Typically made from concrete, or earlier from long stones (often granite), the purpose is two-fold: first as a gutter for proper drainage of the roadway, and second for safety, to keep motorists from driving onto the shoulder, median, or sidewalk.

As a gutter, they guide water from rain and melted snow and ice into the storm drains, so that it does not pool-up on the surface. Large puddles like this can be dangerous, as they can cause hydroplaning at higher speeds and loss of control of an automobile, resulting in a car crash. Even at lower speeds, water and mud can spray up from tires onto motorists behind, causing them to lose visibility. Finally, they can also splash pedestrians, potentially leaving them soaking wet.

As a traffic control measure, they keep motorists on the road, and prevent them from driving around other traffic as if it were a hard shoulder. There is also a disadvantage to this, as it is more difficult to get a vehicle off of the road in case of a breakdown or other emergency.

There is also an aesthetic aspect, in that curbs look more formal and "finished". Since curbs add to the cost of a road, they are generally limited to urban and suburban areas, and are rarely found in rural areas except where certain drainage conditions (such as mountains or culverts) make them necessary.

Curbs may be squared-off, angled, or rounded. The latter kind is most often used at driveways, and continuously along suburban residential streets where there are many driveways, and the sidewalk has a grassy setback from the street. This rounded type starts out nearly flat like the road, curves up in a concave manner to a gentle slope, then curves back in a convex manner to nearly flat again, making it far easier to drive over. The angled type is most often used on major suburban thoroughfares, and is more modern than the other two. The square (90°-edge) type is still almost always used in towns and cities, as it is a straight step down and thus less likely to be tripped-over by pedestrians.

In addition to driveways, curbs also slope down to street level at crosswalks and other pedestrian crossings. This is not only somewhat easier for those on foot, but especially so for those in wheelchairs, as it would be nearly impossible to traverse the curb otherwise.

Depending on the area, the white line that normally indicates the outside (shoulder) edge of the road may or may not be present where there is a curb.

[edit]

Extensions

A curb extension marked by darkened tarmac and black posts

A curb extension (or also kerb extension, bulb-out, 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.

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).

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 protection for pedestrians (particularly children) when leaving premises. The curb extension contains a pedestrian barrier, prevending 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.

[edit]