Why Do Headlights Stay On—For All Cars?

[PianoAl]

Sometimes I need to turn the headlights on despite sunny weather (in highway safety zone, for example), but if I do, the car warns me to turn them off when I'm ready to leave the car (as with most cars). It doesn't just shut them off.

I would like set a mode such that the lights will go off automatically when I leave the car (like walkaway autolock).

Here's my general question: Why do most cars leave the headlights on when you switch off the car? It's been like that since the fifties, and it has resulted in millions dead batteries and many man-hours of inconvenience.

Is there any reason why headlights can be on when the key is removed?

 

[towndrunk]

They stay on so you can safely find your way into the house.

I've always turned it off when possible. I don't need it.

 

[KrenGrl]

but on my car, they do go off after a couple minutes.

 

[PianoAl]

They stay on so you can safely find your way into the house.

I've always turned it off when possible. I don't need it.

Actually, I mean the ones that stay on permanently. Like, "To the owner of the red Ford in front of the store. Your lights are on."

 

[Need]

They stay on so you can safely find your way into the house.

I've always turned it off when possible. I don't need it.

Yeah, because not everyone get to park inside the garage! My previous car (Lexus) has a 60 seconds stay on headlight as default. But I customized it to immediately turn off after I open and close the door.

 

[Need]

Actually, I mean the ones that stay on permanently. Like, "To the owner of the red Ford in front of the store. Your lights are on."

Probably because older cars do not have circuitry connecting the headlights to anything. It is just a electrical light switch.

 

[PianoAl]

Probably because older cars do not have circuitry connecting the headlights to anything. It is just a electrical light switch.

Right. But the radio goes off when you turn off the key, why not the headlights?

 

[Need]

Right. But the radio goes off when you turn off the key, why not the headlights?

That's a good one. I have no answer for that LOL. Wait... I got one. When you are parking in the middle of the desert and you need light to dig a grave....

 

[PianoAl]

but on my car, they do go off after a couple minutes.

Well that's interesting. I just went out to mine, turned the lights to On, got the warning, and went inside. The car locked itself.

Twelve minutes and counting, and the lights are still on.

ETA: After fifteen minutes, I switched them off.

 

[PianoAl]

That's a good one. I have no answer for that LOL. Wait... I got one. When you are parking in the middle of the desert and you need light to dig a grave....

Right, but I always leave the key switched on in that situation. Ain't nobody gonna steal your car while you're burying a body, right?

 

[Shewillbemine]

Weird. I like headlights on at all times. I think it makes the car look "sportier" in addition to the increasing safety, of course.

 

[jrweiss98020]

To answer your original question, I suppose the combination of always-on DRLs (or "signature lights") plus the Auto Headlight function serve well for 99% of the population.

A good question for the local cops would be whether DRLs are legally adequate in a "highway safety zone". If so, you don't have to turn on the headlights...

 

[PianoAl]

I see a lot of cars in the safety zone with just parking lights on (illegal).

Thanks, I didn't realize that the DRLs are on all the time. That's probably enough to satisfy the police, who probably don't care anyway.

The manual says this:

​

Which kinda implies that the car will decide about the DRLs based on which market region or road it's on, but that's probably just poor wording.

But your reply solves it for me, as I no longer need to set them to On.

 

[AdamMacDon]

Right. But the radio goes off when you turn off the key, why not the headlights?

It's a legacy from the old days. And some new cars with old tech may still have it. Lights are controlled by the LCM, which generally has no wiring to the ignition switch. Not that it'd be that difficult to add, but they don't for whatever reason (if it ain't broke, I suppose). For parking lights I can see the reason why the stay on, it's the whole point of parking lights after all! A big problem with most ICE cars is they developed over decades and these systems get kind of thrown into a car rather than designed intelligently from the ground up, like how Tesla did the Model 3. Keep in mind at one point headlights and wipers were an option!

 

[TydalForce]

FWIW, switching the headlights off with the car isn't unheard of; Subaru's been doing it that way since at least the 1980s.

 

[derotam]

Well that's interesting. I just went out to mine, turned the lights to On, got the warning, and went inside. The car locked itself.

Twelve minutes and counting, and the lights are still on.

ETA: After fifteen minutes, I switched them off.

So if you are talking about a car that does NOT have an Auto setting for the headlights, then I agree in the wondering why they didn't just have them switch off when you turn the car off.

BUT if the vehicle has an AUTO setting, and you purposefully turn the headlights to ON, then that's what they are going to do, turn on and stay on... Lots of reasons why someone could want to make sure the headlights stay on when they are not in the vehicle.

 

[PianoAl]

On another forum, I found a possible answer as to why, for decades, headlights don't switch off when you turn the key to off.

In the old days (and in cold weather), a car might have trouble cranking the starter motor if the headlights were on. Thus, if they switched off when you turned off the key, they would come on when you turn it back on—when you start the car.

Googling found:

Why were car headlights ever designed to stay on when the key's removed?

The first cars had gas powered headlights and many early cars that had electric lights the lights were an extra cost option (in as much as anything was standard at that time). So it wasn't that they were designed seperate as much as they were seperate or installed later.

Once lights became standard the draw of the lights was much higher than the switched ignition components. Because the contacts required were so much larger it made sense to keep them discrete from the ignition switch.

And there were other reasons to enable lights to come on independent of the engine running state. EG: turning your lights on was how you called for car hops.

But mainly automotive electrics live a very hard life. They are exposed to vibrations, extremes of heat and cold, extremes of humidity; and all sorts of harsh chemicals. Engineers had to spend a lot of time getting them reliable (not always successfully *Cough* Lucas *Cough*) and any sort of integration with other functions is/was troublesome. Keeping things separate was easier and more reliable. This has only recently changed with microprocessor control of car systems. Even then it is often advantageous, for example, to have components be vacuum actuated rather than electric.
posted by Mitheral at 2:09 PM on November 30, 2015 [4 favorites]
​

and

Before the driver's station trunk release was invented, you often needed the keys for the trunk. Sometimes you wanted the lights to stay on while you did that.​

 

[XLR82XS]

Well that's interesting. I just went out to mine, turned the lights to On, got the warning, and went inside. The car locked itself.

Twelve minutes and counting, and the lights are still on.

ETA: After fifteen minutes, I switched them off.

Your Tesla? My Model 3 exterior lights go off a few seconds after shutting the driver door and walking away.

 

[drtimhill]

Sometimes I need to turn the headlights on despite sunny weather (in highway safety zone, for example), but if I do, the car warns me to turn them off when I'm ready to leave the car (as with most cars). It doesn't just shut them off.

I would like set a mode such that the lights will go off automatically when I leave the car (like walkaway autolock).

Here's my general question: Why do most cars leave the headlights on when you switch off the car? It's been like that since the fifties, and it has resulted in millions dead batteries and many man-hours of inconvenience.

Is there any reason why headlights can be on when the key is removed?

I think many (most) modern cars do turn the lights off after some period for this very reason. My last three certainly did. I suspect this announcements are either for older cars or for people noticing the car lights on before they have timed out and turned off.

 

[Beardley]

Sometimes I need to turn the headlights on despite sunny weather (in highway safety zone, for example), but if I do, the car warns me to turn them off when I'm ready to leave the car (as with most cars). It doesn't just shut them off.

I would like set a mode such that the lights will go off automatically when I leave the car (like walkaway autolock).

Here's my general question: Why do most cars leave the headlights on when you switch off the car? It's been like that since the fifties, and it has resulted in millions dead batteries and many man-hours of inconvenience.

Is there any reason why headlights can be on when the key is removed?

I have mine set to auto, and they turn off a minute after I walk away and the car locks. If you need them to turn on for a brief period of time, (not sure what a highway safety zone is) you can just do a quick flick of the washer fluid / wipers. The wipers activate the headlights, from my experience.