When is it safe to use high-beam headlights when other vehicles are on the road in front of you?

Answer 1

Every US state and every Canadian province has its own laws regarding when you must dip to low beams in terms of distance to a leading or oncoming vehicle. These were set before the maximum allowable high beam intensity (regulated at the Federal/national level) was doubled in 1978. When that happened, the dipping distances to prevent blinding oncoming and leading motorists should have been increased. Most states have 500 foot (oncoming), 200 foot (leading) dipping distances specified in their laws. Because the illumination at the eye is proportional to the lamp's intensity and inversely proportional to the square of the distance, a solid estimate can be made for how dipping laws should have been changed. If 500/200 feet were deemed to be acceptable for the pre-1978 headlamps, then for the 1978-up headlamps the dipping distances should have been changed to 700/280 feet, and for today's highest-intensity lamps the dipping distances should be 970/390 feet. The US National Highway Traffic Safety Administration (Federal DOT) and its Canadian equivalent Transport Canada, can alter headlamp technical specifications but cannot alter state/provincial use-of-lights regulations, nor can those national agencies compell state/provincial lawmakers to change the rules regarding dipping distances.

So if you want the highest chance of being *legal* without looking up the rule for each state/province you drive in, then dip from high to low beam when within 500 feet of an oncoming car, or 200 feet of a leading car. If you want to be *safe*, then about 800 feet (oncoming) or 300 feet (leading). This means you will be using your low beams most of the time unless you are driving in a rural area or another area with very sparse traffic. Also, do not use the high beams in rain, fog, snow, or daytime.