Why should a driver turn on headlights, using low beams, rather than relying only on identification or clearance lights, when visibility is poor at dawn, dusk, or in rain or snow?

Explanation

A driver should turn on headlights, using low beams, when visibility is poor at dawn, dusk, or in rain or snow, because headlights make the vehicle far more visible to other drivers who are also struggling to see in those conditions, while low beams avoid the glare that high beams can cause for oncoming or nearby drivers even during the day. Communicating presence is part of the required knowledge CDL applicants must have under 49 CFR 383.111(a)(8)(ii), but that paragraph only names it as a required knowledge topic; it does not itself specify using low beams in reduced-visibility daytime conditions, which is standard driver-training guidance rather than a codified federal rule. Identification and clearance lights remain legal at any time of day, headlight use is not limited to hours after sunset, and fuel efficiency has nothing to do with the choice between low and high beams.

Source: 49 CFR 383.111(a)(8)(ii)