Ask The Driving School Instructor: Overdriving Your Headlights
September 2, 2014
Question: What do they mean when they talk about overdriving your headlights?
Answer: Overdriving your headlights means driving too fast to be able to stop in the distance lit by your headlights. Automobile headlights are only effective for a certain distance. On average, with low beam headlights, you can only spot objects in the road for a distance of about 160 feet in front of your vehicle. With high beams, you can see about 450 feet ahead. Those distances don’t take into account the age, visual abilities of the driver, or road conditions. Your headlights can’t light objects over hills, around curves, or dips in the road and they’re even less effective in rain, fog or snow.
Let’s say you’re driving at 40 mph on a dark road using your low beam headlights when you see an object in the road ahead. On average, once you see the object ahead, it will take you 1.5 seconds to fully react and hit the brakes. Once you hit the brakes, it will still take time for the vehicle to come to a complete stop. At 40 mph, it takes approximately 189 feet to bring a car to a complete stop. Remember that your headlights only allow you to see 160 feet ahead so, at 40 mph, you won’t be able to stop in time to avoid hitting the object in the road.
When driving on a dark road, it’s best to use your high beam headlights but the law requires you to dim your headlights within 500 feet of an oncoming vehicle. Remember also that the glare of the headlights from an oncoming vehicle can blind you to objects behind that vehicle. When driving in rain or snow and especially in fog, it’s best to use your low beam headlights. In foggy conditions, your high beams will only reflect back off of the fog and make it even more difficult to see ahead.
Speed limits are posted for ideal conditions such as a clear day on dry roads. At night, no matter what the posted speed limit may be, it’s best to slow to a speed where you can stop your vehicle within the space lit by your headlights.