speed limits are a fixed point on a continuously variable scale. They do not vary with weather conditions.
When the conditions are inclement to driving at certain speeds under the limit (fog, torrential rain, flooding, ice on the road, twenty tonnes of spilled fudge topping, mass migration of camels), the traffic constabulary has the legal authority to infract you for "reckless driving".
Posted limits offer guidance in best conditions, not instruction under any condition. Conditions can and do demand that the vehicle operator use their faculties to determine a safe-for-driving pattern of behavior; the body of laws which include, but is not limited to, speed limits does the same.
This exchange began with the phrase "safe driving speeds". I do not appreciate the substitution of a reference to a body of laws, which must deal with changing situations, with a proper subset, which only applies to certain situations, in order to contradict the analogy which this exchange also epitomises.