Per the CDL manual, within how many feet of an oncoming vehicle, and within how many feet of a vehicle you are following, should you dim your headlights at night?

49 CFR 383.111(a)(11); AAMVA CDL Manual (nhcdm.pdf) Section 2.11.5, p. 2-27, 'Avoid Blinding Others'

Glare from high beams can blind another driver for several seconds, and the manual notes that at 55 mph a vehicle can travel more than half a football field's length while a driver recovers from glare blindness. The 500-foot dimming rule, applied both to oncoming traffic and to vehicles being followed, is a concrete number the knowledge test expects drivers to know rather than a vague 'dim when it seems polite' guideline.