Why does the guidance for driving in fog say to slow down before entering a fog bank, rather than waiting until visibility is already reduced?

Explanation

A driver should slow down before entering a fog bank, not after, because fog can appear suddenly and visibility inside it can deteriorate rapidly, so reacting only once already inside leaves far less room to adjust safely. Fog is not named anywhere in the required knowledge topic for extreme driving conditions, 49 CFR 383.111(a)(12), whose examples are bad weather (snow, ice, sleet, high wind), hot weather, and mountain driving; the closer match is the broader safe-operations umbrella at 49 CFR 383.111(a)(1)(ii), "procedures for safe vehicle operations," which points generally at driving safely rather than naming any specific technique, so this slow-down-in-advance practice is standard driver-training guidance rather than a codified federal rule. The other options fail for a shared reason: each invents a requirement or benefit, a federal speed limit tied to fog signage, reduced engine wear, or a reason to close the windows, that has nothing to do with why slowing down early actually improves safety.

Source: 49 CFR 383.111(a)(1)(ii)