A curb or kerb (see spelling differences) is the edge where a raised pavement/sidewalk/footpath, road median, or road shoulder meets an unraised street or other roadway. Typically made from concrete, asphalt, or long stones (often granite), the purpose is twofold: first as a gutter for proper drainage of the roadway, and second for safety, to keep motorists from driving onto the shoulder, median, sidewalk, or pavement.
As a traffic control measure, curbs keep motorists on the road, and prevent them from driving around other traffic as if it were a hard shoulder. The disadvantage is that it makes it 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 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 are not universally used, however, even in urban settings. See more at living street.
In addition to driveway aprons, curbs also slope down to street level at crosswalks and other pedestrian crossings. This is called a curb cut (U.S.), dropped kerb (UK), pram ramp (Australia), or dish (Ireland). This makes it somewhat easier to traverse for those on foot, but especially so for those in wheelchairs, or for people with prams and strollers as it would be difficult 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.
Typical types of curb based on the cross sectional shape: insurmountable, rolled, or rounded (used in many residential areas), surmountable or traversable (used along islands at intersections allowing errant vehicles to cross), and barrier ("L" shaped and used as a boundary to prevent vehicle from exiting the pavement).
Curbs may be squared-off, angled, or rounded. Rounded curbs are 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 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. Thus, they are also known as mountable curbs in some localities. 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.
Curbs may be constructed of many materials but most often are made of concrete or asphalt. The type of material may depend on the type of paving material used for the road and the desired function or need. For example, a concrete curb used with an asphalt pavement provides a highly visible barrier at the edge of the pavement. Other types of curb material include stone slab, cobblestone, and manufactured pavers.