Curb (road)

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A curb or kerb (see spelling differences) 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 accumulate 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 (of course when the drains get blocked these water routing systems cause a massive puddle to form around them ensuring that pedestrians in the area are very likely to get soaked).

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 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.

Curb (road)

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See also