Why should a driver use low-beam headlights, not high beams, when driving through fog?

Explanation

A driver should rely on low beams, plus fog lights if the vehicle has them, rather than high beams when driving through fog, because high-beam light reflects off the water droplets that make up fog and bounces back toward the driver, reducing the driver's own forward visibility rather than improving it, while low beams and fog lights light the road ahead with far less of that glare-back effect. 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 headlight-reflection fact is standard driver-training guidance rather than a codified federal rule. The other options fail for a shared reason: each invents a federal restriction or mechanical effect, a visibility-based high-beam ban, bulb-life savings, or an automatic daytime-running-light interaction, that does not actually explain why high beams perform worse than low beams specifically in fog.

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