Per the CDL manual's guidance on matching speed to the road surface, how much should a driver reduce speed on a wet road compared to normal dry-road speed?

49 CFR 383.111(a)(9); AAMVA CDL Manual (nhcdm.pdf) Section 2.6.2, p. 2-14, 'Slippery Surfaces'

The manual gives drivers a concrete, memorable rule of thumb, roughly a third off on wet roads, half or more on packed snow, and a crawl on ice, so speed decisions in changing weather aren't guesswork. This graduated scale is a common source of knowledge-test questions because it tests whether an applicant understands that different slippery conditions call for different degrees of caution, not a single blanket rule.