See: Speed limit
Used to indicate a maximum speed limit.
Amongst one of the most familiar signs, this sign is used where parking or stopping should be prohibited. Usually shown as a red cross inside a blue circle with a red ring in Europe, and a 'P' in a red circle with a cross through in North and South America, Asia, Australia, Africa and Ireland.
Either overtaking is prohibited for all vehicles or certain kinds of vehicles only (e.g. lorries, motorcycles, etc...) In the USA this is usually phrased as "no passing zone" and indicated by triangular signs on the opposite side of the road (where they will be seen by a driver who is attempting to pass, and sometimes supplemented by a rectangular, black-on-white sign on the right side of the road that says "Do not Pass") and/or by a double solid line painted down the middle of the road. Conversely, when the no passing regulation is lifted, a rectangular, black-on-white sign that says "Pass with care" is placed on the right side of the road and the centerline changes to either a single broken centerline (indicating that passing is allowed in either direction) or a combination of a broken and a solid centerline (indicating that passing is only allowed from the side with the broked centerline).
Either for all vehicles or with some exceptions (emergency vehicles, buses). These are usually to speed up traffic through an intersection or due to street cars or other right of ways or if the intersecting road is one-way.