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Curbs from WikiPedia:

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 prevent motorists from driving onto the shoulder, median, sidewalk, or pavement.

Curbs guide water from melted snow, rain, and ice into storm drains, so that it does not gather on the road. Big puddles can be dangerous, because they can cause hydroplaning at higher speeds and loss of control of a vehicle. At lower speeds, water can spray from tires onto following vehicles, causing poor visibility. Also pedestrians can be splashed by water.

Curbs also keep vehicles on the road and prevent people from using sidewalks as a hard shoulder.

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