[Skyline_GT]

While it's an innovative way to use the headrest to break open the glass in case of emergencies but i doubt if they were made detachable specifically keeping such scenarios in mind. If they had been designed for such purposes too then why would some of today's budget cars get non detachable ones?
Also in most emergency situations one can always recline a bit on seat and use his/her legs to safely break open the window pane. Shouldn't take more than a couple of kicks with shoes on to break it open. Unless if the car is submerging in water then the outside pressure will make it extremely difficult and dangerous to break the glass open.

[hakunabatata]

Your theory is not at all wrong, in fact it's a great idea and can be implemented in future cars with that very intention. It might require some changes as Car windshields and windows are made from tempered glass. Tempering puts the outer surfaces into compression and the inner surfaces into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards.

The tempered panels are very tough and can withstand hits from a few small stones, cricket balls etc. Because of the nature in which the stress is spread out in the glass, it can sustain impacts from object with a large surface area, say a tennis ball which might compress on impact on the glass thus providing more contact surface area. In such cases the force of impact is spread out evenly.
In order to break the glass, one needs to apply a large amount of force in a very small area, somewhere close to the size of a pen tip.

This is the primary fact that makes it plausible to use the detachable headrests, as the connecting rod has a sharp point. One can use it to poke a hole in the window. If a headrest is designed properly, such that it can deliver the necessary blow when needed, then this could certainly improve the safety of future cars.

And as Skyline correctly put, it will be impossible to open the doors/ roll down the windows if the car happens to fall in to a water body and is submerged to a significant depth. The water pressure at that depth would make it very tough to get out of the car. It could be very handy to have a built in tool to facilitate escape in such situations.

Nowadays there are fancy multi-tools available which have a few safety tools built in to them (Emergency hammer + Belt cutter + Torch). It is always a good idea to carry one in your car. Better safe than sorry.

[sridhu]

In a lot of new cars, there is a sharp nub on the seat belt latch for this very purpose

[gthang]

Didn't think that detachable headrests, a misnomer, for it is not for resting your head, but for restraining your neck/head, were made as such with detachment in mind rather than height adjustment.
It just so happened that some "MacGyver" dude figured out that it could be used as a tool.
Budget cars have non adjustable restraints because of budget. They are cheaper to make.

Social menacing network can help you put 2+2 together and come out with 15.

Cheers.

[bhandary]

There are some cars with fixed headrest. Does this mean that there is one less safety tool inside the car? I keep the jack lever in the driver door pocket for any emergency.

[Ravindra M]

I have not seen any car owner's manual which state this fact. If the headrests are meant for this, the manufacturer would definitely mention it in the safety aspects of the owner's manual. Hence I think we can use it whenever and wherever possible and feasible.

[phamilyman]

Ebert Hubner in a 2006 training told us that you can sometimes escape a stranded (flooding) car by kicking out the front wind screen by both legs. I've never verified it but it makes sense. Even smashing the glass isnt easy - plus to make a hole large enough for you to escape safely will take time.

[Latheesh]

I have something like this in my car (hammer and seat belt cutter)