What speed law might make driving at the posted speed limit illegal in bad weather? Thx!

In a "presumed" speed law defense, you (the defendant) have the burden of proving your speed was safe and prudent. In other words, the speed law presumes the posted speed limit is the fastest safe speed. It is up to you to prove that going faster at the time you were ticketed is also safe. Meaning that if you are going at the speed limit in bad weather, you can be given a ticket.