According to the CDL manual, what is the best advice for driving in fog, and if driving is unavoidable, what should a driver do?

49 CFR 383.111(a)(12); 49 CFR 392.14; AAMVA CDL Manual (nhcdm.pdf) Section 2.12, p. 2-27, 'Driving in Fog'

Fog removes the visual cues drivers normally rely on to judge distance and closing speed, which is why the manual's first recommendation is avoidance rather than a driving technique. When driving through fog can't be avoided, the specific combination of low beams/fog lights, 4-way flashers, and not stopping on the roadway is what keeps a stopped or slow-moving commercial vehicle from becoming an invisible hazard to traffic behind it.