The law is the law. The speed measurement equipment manufacturers love them too. Their products work perfectly in an atmosphere of absolute statutory certainty. The mind-set that supports statutory speed limits doesn't like to admit the existence of the gray areas where evaluation and judgement take precedence over rote mechanical performance. Statutory speed limits are not by definition absolute.
Statutory limits apply in the absence of posted speed limits.
Statutory limits can be absolute or prima facie. If the police couldn't prove that speed was unreasonable for conditions, then they hadn't proved it was illegal. And the presumption of innocence required them to prove it was unreasonable. Speed limits prior to 1974 were not all prima facie, this varied from state to state. The author of the above article is mistaken in the application of prima facie speed limits.