I just happened to take a look at the sidewalk in a city setting, and I realized the curb is NOT the edge of the sidewalk, but a separate component. But, why is the curb necessary? What a waste of labor and materials when the sidewalk itself, by its very existence in a 3-D world, HAS an edge…it doesn’t need the addition of a curb! (Take a good look at a city sidewalk where the curb butts up against the sidewalk…as opposed to being divided by a slim section of grass or trees, for example.)
Any clues? WAGs?
The curb normally incorporates a gutter, which must be laid out for proper flow; a curb needs a curved edge and a finished face, too. This entirely different structure requires a different sort of framing and finishing than a sidewalk, and it’s easier to do this in separate steps. Also, not every curb is contiguous with a sidewalk; if the curb were part of the sidewalk, the non-contiguous portions of curb would have to be framed and poured separately, and the curb wouldn’t be continuous.
In this detail for a residential street, you can see that the sidewalk concrete is only four inches deep; the curb is eight inches deep with compacted road base (gravel, more or less) beneath it. It would cost several times as much to make the sidewalk to the curb’s standards. You would also have to make the sidewalk a lot wider to allow pedestrians to walk a safe distance from the street which would also add to the cost.
You can have curbs without sidewalks, too. The street I live on has curbs and gutters, but no sidewalks. Actually, I can’t think of any streets that don’t have curbs but do have sidewalks. Perhaps there are many streets, especially in residential areas, where the curbs are put in years before the sidewalks are, if the sidewalks are ever put in at all. Therefore, the curb is not just an edge for the sidewalk, and has its own purpose. My WAG–it helps define the gutter, and it gives a more finished look to the street. It might also keep the dirt from shifting and pushing a sidewalk into the road.
Curbs may not be absolutely necessary. The street my parents live on doesn’t have them (or sidewalks), and they’re just fine. Of course, the street they lived on before that wasn’t even paved, so that’s not necessary, either.
The curb isn’t the edge of the sidewalk, it’s the edge of the street. The confusion may come from the fact that the curb and sidewalk are at the same height.
They do make curbs and gutters that are incorporated into sidewalks. I think they are called Hollywood style (don’t quote me).
The face of the curb is at about a 45 degree angle. The angle accomodates cars going into driveways. Still, its a pretty good bump. The sidewalk part is usually only about 2 feet wide. They are done in one pour. You can find these all over Denver Colorado.
To answer the OP:
The reason curbs are built is for erosion. If there are no curbs, rain accumulates on the road surface and washes off the side of the road, eroding the softer surface it finds there. When there is a sidewalk the rainwater gets trapped between them, and may erode the space into an unsightly ditch. A curb “curbs” the erosion, channeling the water into a gutter to drain away.
Curbs are used in developed areas because with all the pavement from roads, driveways, sidewalks, and many buildings the erosion process happens much faster.
IANA engineer, but this is fairly straightforward.