The 'basic speed law' in most states means:

The 'basic speed law' (or 'reasonable and prudent' law) requires drivers to operate at a speed that is reasonable and safe for current conditions — even if that speed is below the posted limit. In rain, fog, snow, heavy traffic, or other hazardous conditions, the safe speed is below the posted limit. A driver can be ticketed for driving 'too fast for conditions' at a speed lower than the posted limit. The posted limit is a maximum for ideal conditions, not a target for all conditions. Conversely, you may not exceed the posted limit even if you feel conditions allow it; the posted limit is also a legal maximum. This dual standard means drivers must use judgment as well as obey signs.