In some states, if not all, the law requires drivers to dim their headlights for approaching traffic within 500 ft (commonly) but only on the same "roadway." The term roadway is not the same as highway. A divided highway consists legally of two separate roadways. Therefore, the laws generally do not require a driver to dim headlights for the oncoming traffic of the opposite roadway.
Many drivers, however, still consider it courteous to dim their lights, even though "high beams" are not nearly as problematic for approaching traffic of divided highways. Others get irritated with drivers who do not practice that courtesy.
Since high beams from cars in the opposite roadway generally do not bother me, I myself do not feel bound to dim mine for them. Those who begin flashing their high beams in an attempt to "educate" me seem a little ridiculous to me, and are probably doing so because they misunderstand the law. Be careful: state traffic laws vary on many specific points such as this, so it is best to know the law for a given state and not to rely on a general answer such as this one. I was unable to find a state-by-state comparison, but inquiring with the state police is a simple enough way to find the answer for your particular location.
Usually dim headlights are caused by a poor ground connection.
the courteous time to dim your headlights is as soon as you see another vehicle, or within one half mile.
when there are oncoming cars, it is appropriate to dim your headlights when they are about 300ft away. Check your diver manual for the most correct answer.
Your headlights may not dim because you don't have dimmer lights. Dimmer lights can blow and leave you with just the high beams.
You should dim your headlights within 500 feet of the car in front of you. The rule of thumb is, if you can see another car, dim your lights.
If you can see the area lit by their headlights, then you are probably close enough to dim your own.
If by "dim headlights" you mean low-beams... the bulbs are bad and must be replaced.
The headlights on the 2002 Toyota Tacoma might work on dim but not on high beam because of a blown fuse. If the headlights are too dim to see properly in the dark, there could also be a problem with the alternator and the charging system in the vehicle.
100 feet
never
you might have a short in your alternator. In my car the headlights would dim when I put on the brakes or tried to use cigerette lighter to power up my phone.
A common reason for dim headlights are failing bulbs that need to be replaced or improper cleaning. It may also be a more serious wiring issue that will need additional work.