The headlights in your car are set according to standards set by the Road Safety Commission. Your headlights are aimed at a certain angle and position to light up a certain portion of the roadway ahead. There are limits to this and are usually set according to the vehicle and jurisdiction involved. Once set the headlights throw a beam of light ahead a certain amount of length. The lighted portion is visible up to a certain length ahead of the vehicle. This is set to normal and within highway speed limits. Over driving means driving faster than time it would normally take to react to roadway variations and any other object on the road itself. The reaction time is much diminished when driving faster. Another variable in this matter of course is the weather, visibility, fog, time of day, condition of driver(fatigued or alert) and other conditions that can affect reaction in time. When you go over that limit you are over driving your headlights and thus put yourself in danger of a collision or accident.