Dear Tom and Ray:
I need your help to settle an argument I have with my no-good brother-in-law from Illinois. He always drives at night with his fog lights on in addition to his headlights. He says he can see better with them on. I tell him, "Sure, but what about the way they blind oncoming traffic?" He says if they were too bright, then it wouldn't be legal to have them.
I tell him, "But all that bright light up close actually reduces your long-range vision."
He says, "How can more light be bad?"
I say, "And it makes your engine work harder, shortening the life of some components and reducing mileage."
He says the car was designed for it, and he's done it for years with all kinds of cars and has never noticed any ill effects. I have fog lights too, but I use them only three or four times a year -- essentially, on deserted county roads in the fog, when they can actually make a difference. I say he's a feckless Bears fan. He says I'm a pansy Packers fan. Which one of us is a bigger jerk?
-- Fred from Wisconsin
RAY: Well, I think the jury's deliberating hard on that one, Fred. But they might be leaning toward you.
TOM: There are two kinds of supplemental lights you see on the fronts of cars. There are fog lights, which are mounted down low and illuminate the road immediately in front of the car.
RAY: You may have noticed that when it's foggy, the closer you get to the ground, the less dense the fog is (and in fact, using your high beams in fog makes it harder to see). So fog lights attempt to help you by lighting up the lowest, closest portion of the roadway.
TOM: And since they're aimed down at the road for 30 feet or so in front of the car, they shouldn't bother oncoming drivers much, if at all.
RAY: So, if he's really using fog lights, you should leave the poor guy alone and get off his case, Fred.
TOM: Sure, he's using a tiny bit more gasoline to power the extra lights, but the difference in mileage is barely measurable. So if he prefers more light to an extra fraction of a tenth of a mile per gallon, he's free under the U.S. Constitution to make that trade-off.
RAY: Now, the second type of lights you see are called "driving lights." Those essentially are an extra set of more powerful high beams.
TOM: And like high beams, those are aimed up and far ahead, to light up as much of the road and roadside for as long a distance as possible. So, driving lights WILL blind oncoming drivers, and they should be used just as high beams are used: on remote roads when there's no oncoming traffic. And they need to be switched off when another vehicle approaches.
RAY: So if he's got high-powered driving lights, and he's leaving those on all the time (even if they're legal and correctly aimed), then he's putting himself and oncoming drivers in danger by making it hard for them to see. And I'd be surprised if he hasn't been given that message by stars-seeing oncoming drivers who flash their high beams at him in annoyance.
TOM: If that's the case, and he's still using his driving lights all the time anyway, then he deserves to be harassed by an annoying brother-in-law. And you sound like just the guy for the job, Fred, so stay on him!