What effect do posted speed limits have on actual traffic speeds?

Posted speed limits have very little effect on actual traffic speeds. There is a common belief that the posting of speed limit signs will cause drivers to react accordingly. This is not true and is why posted speed limits must be realistic to achieve compliance.

Unrealistically low speed limits invite violation even by responsible drivers. Enforcement of unreasonably low limits sets up the so-called "speed trap," which results in poor public relations. The posting of proper speed limits has the beneficial effects of smoothing traffic flow and aiding effective law enforcement.