As pertains to Oregon:

Violation of Basic Speed Rule (811.100)
(1) A person commits the offense of violating the basic speed rule if the person drives a vehicle upon a highway at a speed greater than is reasonable and prudent, having due regard to all of the following:
(a) The traffic.
(b) The surface and width of the highway.
(c) The hazard at intersections.
(d) Weather.
(e) Visibility.
(f) Any other conditions then existing.     

All travel on public streets and highways is subject to the Basic Speed Rule. The Basic Speed Rule states that a motorist must drive at a speed that is reasonable and prudent at all times by considering other traffic, road and weather conditions, dangers at intersections and any other conditions that affect safety and speed. In other words, drivers are expected to use good judgment in selecting their speed.
The Basic Speed Rule does not authorize a driver to exceed the posted speed. If a motorist is charged with violation of the basic speed rule, the posted speed becomes the primary evidence that he/she exceeded the Basic Speed Rule. In this case, it would be up to the motorist to prove that he/she was driving in a reasonable and prudent manner for the conditions.

A person can also drive below the posted speed and violate the Basic Speed Rule. For instance, if there is ice or snow on the roadway, a driver can be traveling less than the speed posted and still be traveling faster than is reasonable and prudent for the conditions.