At night, why should a driver slow down enough that the vehicle can stop before reaching the edge of what the headlights actually illuminate?

Explanation

A driver should slow down at night so the vehicle can come to a stop before reaching the edge of what the headlights actually illuminate, because driving faster than that removes any margin to react once a hazard comes into view. Low beams typically light only a few hundred feet of road ahead, so overdriving that range means a driver could spot a hazard too late to avoid it. Night driving factors that affect a driver's ability to see, including limited headlight range, are a required CDL knowledge topic under 49 CFR 383.111(a)(11), but the regulation only names this as a topic area a test must cover; it does not itself set a specific speed rule, which is standard driver-safety guidance rather than a codified requirement. The other options fail for a shared reason: there is no separate federal nighttime speed limit for commercial vehicles, no speed-based restriction on using high beams, and insurance pricing has no bearing on the underlying safety principle being tested.

Source: 49 CFR 383.111(a)(11)(iii)