What does the 'basic speed law' require of drivers, regardless of the posted limit?

The basic speed law requires that you never drive faster than is reasonable and safe for the existing conditions — traffic, weather, visibility, and road surface — regardless of the posted speed limit. The posted limit is the maximum under ideal conditions, not a guarantee that the limit is always safe. In rain, fog, heavy traffic, or near hazards, the safe speed may be well below the posted number, and driving too fast for conditions can be a violation even when under the limit. Understanding that you must adjust to conditions, not just obey the number on the sign, is a fundamental rules-of-the-road concept.