At what speed can hydroplaning begin to occur on a wet road, according to the CDL manual?

49 CFR 383.111(a)(9); AAMVA CDL Manual (nhcdm.pdf) Section 2.6.2, pp. 2-14 to 2-15 (the 'Hydroplaning' subheading starts on 2-14, but the specific '30 mph' figure itself appears on 2-15)

Many drivers assume hydroplaning only happens at highway speeds in heavy storms, but the manual's 30-mph threshold means it can happen in everyday wet conditions well under typical speed limits. Recognizing standing water, tire splash, and clear reflections on the road ahead, and slowing accordingly, is a basic defensive habit the knowledge test expects every CDL applicant to know.