At first I thought maybe it should be in GQ’s as ‘when should you turn on your headlights?’ but it’ll attract more car guys here.
When street lights go on is generally when I turn mine on but I think thats turning out to be way too late these days in the USA.
It depends on the weather. On a dark cloudy day sometimes I’ll have them on at noon. Anytime there is preciptaion I have my lights on. I think it is a law in Wisconsin that if your windshield wipers are on your headlights have to be on too.
I usually judge when I should turn my lights on by noticing how well I can see other cars. If I have difficulty seeing a car in the distance, due to darkness, I’ll turn mine on. This is around an hour before sundown, or a little earlier.
I have noticed times when the light is not good but not too bad either and most folk will have their lights on.
Under that situation it makes sense to have your own lights on too as everyone is looking for them.
I’ve seen a few near misses because someone was driving around with no lights and maybe it was down to them being in a small minority.
If I am between the low sun and the oncoming traffic, I usually put my lights on so they can see me despite the glare. At this time of year, that’s usually around 3 p.m. Otherwise I put my lights on when I notice that about half of the other cars have theirs on.
I turn them on when I have a hard time seeing the white lines on the road. However, on occassion I do turn them on when I think the people around me can’t see me easily.
My lights are always on.
My car, 2000 Celica GT-S, has daytime running lights (which I have no control over) and a sensor that automatically turns on the regular headlights when it gets dark enough. They even come on if I stop in the shade (like under a bridge) for a few seconds. It was annoying at first but you get used to it.
In other cars I usually turn them on when I have trouble seeing or when I notice that everyone but me already has them on.
[hijack]
bafaa, how do you like your Celica? Is there any room in the back seat?
I read in a car magazine that driving with your headlights always on cuts down on your chances of having an accident, so I’ve been driving with my headlights turned on for the past four years since getting my ‘new’ car. [it was a '95, but it WAS new to me. ;)]
However, I’m forever getting drivers who ‘flash’ me, as if I didn’t know my light were on, and I just wave…
I LOVE my car! It’s loads of fun to drive.
The back seat is great for holding cd’s and small change.
I play it by ear–or eye, rather–but the older I get, the more likely I am to turn 'em on early.
My car is black and pretty small (it’s a Cavalier Z24, and I live in a heavy population of big-ass pickup trucks) so I put on my headlights as soon as it starts getting yellow. You know what I mean – that time in the late afternoon when the sun’s setting and it comes in at an angle and blinds you, but the light is yellowish? I drive on a lot of wooded streets so I put my lights on to make it easier to be seen in the shadows.
I leave them off. Then when someone flashes their lights at me I chase them down like the dog they are with the rest of my gang members. Then we steal their kidneys, infect them with HIV via a pin, and tattoo them with the three words in the English language that end in “gry”.
I’m sure you’ve all heard about me and my friends…
I like Andygirl’s response.
I turn mine on when the clear majority of other drivers have theirs on, though I wish most of them would wait longer before thinking it’s dark.
I turn them on when I start wishing that the oncoming traffic would turn on their headlights.
At twilight, or when it’s raining. You know, when I can’t see very well. 
When I go to work, and then when I come home from work.(in my big-ass pickup truck.)
Safety tip:
LEAVE YOUR HEADLIGHTS ON AT ALL TIMES! (Or have daytime running lights.) It vastly increases your car’s visibility to other drivers.
Get used to turning them on every time you drive. It’ll make an accident just a little bit less likely. Take it from RickJay, who works in the vehicle safety field, okay?
Drive safe.