So-called ‘passive safety’ systems in cars do nothing … until you crash. Then they spring into action literally faster than the blink of an eye, with the express intent of saving your neck. They’re called passive because no action or intervention by the driver brings them into play.

Perhaps we should back up a sec. Modern cars are jam-packed with safety kit, which is split into two fundamental camps – ‘active’ safety systems, like brakes, which are designed to help you avoid crashing in the first place, and ‘passive’ safety systems, like airbags, designed to lessen the impact on you when you are actually crashing. Passive systems are invoked automatically by the car.

Australia has the dubious distinction of being a developed country with one of the world’s oldest fleet of cars actually ‘out there’ on the road. Average age: about 10 years. In terms of technology this is a bad thing – it means the average Aussie driving around today is 10 years behind the eight-ball on safety technology (and emissions technology, and…) It’s easy to forget that if you’re a motoring journo, swanning around in new cars all the time.

So, although many high-end Euro luxury/performance cars will offer the lucky few who buy them something like nine airbags, the average Australian in a car today is lucky to be protected by just two – at the front.

It’s interesting to note that while airbags are passive devices, seatbelts are active – because the driver must elect to use them. Thankfully we’ve been pretty good at that in Australia since the 1970s, and it remains the main reason why our road toll, per capita, is much better than that of the USA.

Airbags are just part of a crash safety system that is fundamentally integrated into the car’s structure – which is why you can’t add them to a car in an aftermarket sense. This is because although the hardware itself (crash sensor; steering wheel with inbuilt airbag module, seatbelt pre-tensioner) could be easily boxed up and sold over the counter at Super Cheap or Repco, you could never tune it so it would work effectively.

Airbags must deploy at the ‘Goldilocks moment’ – not too early, and not too late. And here, a few milliseconds either way makes the difference between life or death. That’s why car companies do all that expensive crash testing. It’s the precise choreography of all that in-car explosive stuff – down to one-thousandth of a second accuracy – that really does the job of saving lives.

The blink of an eye takes about a fifth of a second (200 milliseconds). The bit of a car crash that can kill you is all over in about 40-80 milliseconds – two-and-a-half to five times faster than the blink of an eye. That’s from the moment the car first contacts the thing it hits, to the point the impact danger is effectively past. In that impossibly small window of opportunity the crash sensor must determine that a crash is taking place, decide if it is serious enough to deploy the airbag, send a pulse to the airbag’s detonator if it is (and a separate pulse at a separate time to the detonator in the seatbelt pre-tensioner). Then both charges in the pre-tensioner have to explode and, in the case of the airbag, generate enough nitrogen gas to deploy the bag, get it out there, ready, fractions of a second before your head hits it. Once the crash sensor says the binary equivalent of ‘go for it’, it’s all over in under 40 milliseconds.

There are variations, but most airbags use a detonator and solid propellant. The detonator fires into the solid propellant, which causes a rapid chemical reaction. An inert gas, usually nitrogen, is created at high pressure. It’s this that inflates the bag. Nitrogen is comparatively harmless, which is a good thing … since ordinary air is about 78 per cent nitrogen. (You can actually suffocate in a 100 per cent nitrogen environment, since there’s no oxygen, but that’s not much of a risk in a car crash.)

Sodium-azide was a popular propellant in older airbags. Unfortunately, it was highly toxic (sodium-azide itself, that is – not the post-deployment byproducts, which were carbon-monoxide and nitrogen oxide). It was mostly phased out in the 1990s. Nitrocellulose (think: gunpowder) has largely also been phased out because it’s not as stable as newer propellants.

If you’ve ever been in a crash in which the airbags deploy, you’ll probably notice the cabin is liberally dusted with white powder. You will be, too. Some people are moved to ponder what it is, and the health implications of breathing this stuff. Don’t worry – it’s not a combustion product. It’s just talcum powder or commercial chalk, which is used to lubricate the bag.

For every car crash, there are actually three big hits – and it’s the third one that kills you. Hit one is between the car and whatever it hits (think: 100-year-old gum tree). In slo-mo, the metal starts to deform. The tree pushes back on the car, which accelerates backwards (decelerates, if you like). You get thrust forward, relative to the car, which leads to…

Hit two, which is between you and the car. If you’re dumb and not wearing a seatbelt, much of that impact will take place between your head and the windscreen; fade to black. If you’re one of the 99-point-something percent of Australians who does wear a seatbelt, the impact will mainly occur between your hips and thorax, and the belt itself (your head and legs might still hit the dash, however, causing life-threatening injuries). If there’s a lot of slack between you and the belt, hit two will be bigger than if the belt is snug – because if it is loose, your body will be travelling at a high speed relative to the decelerating car when you slam into the belt. But relax; you’re not dead yet.

Hit three is the one that really counts. It’s the arbiter of life and death. After your body hits the seatbelt, your internal organs slosh forward, and hit the front of your rib cage (on the inside). If that hit is big enough, it can sever your aorta, the major blood vessel in your chest, or rip a hole in your heart. That’s bad. If your head hits the dash (or the windscreen) your brain will hit the inside of your skull. Blood vessels will tear, and blood will leak into your skull cavity, which is, essentially a rigid box. The pressure of the blood inside your head – there’s nowhere for it to go – can kill you. (Also bad…)

This is what all that crash-mitigation technology seeks to avert – in simplistic terms the airbag springs into action between collisions one and two, which has a flow-on benefit to collision three.

Look at it like this: A crash sensor, which is just a box with an accelerometer in it (or accelerometers, if the car has side airbags), and a chip inside which says ‘go bang if the crash gets worse than this XXX’. If the crash severity exceeds this predetermined line in the sand, the seatbelt pre-tensioner fires off, sucking in all the slack in the belt (reducing collision two). It also sucks you square-on into the seat so you present a better target to the airbag. Within instants the airbag is full. It actually starts deflating before your heads hits it – when that happens, it’s the automotive equivalent of jumping off a four-storey building onto a stack of Sealy Posturepedic mattresses, as opposed to, say, the bare concrete footpath.

There’s one more thing happening: the car’s structure itself is protecting you. All that controlled deformation in engineered ‘crumple zones’ up the pointy end of the crash is absorbing energy – before it gets to you. This is why old crashed cars never looked that damaged, and yet the people inside died. These days, the cars often appear royally screwed over by similar collisions, and yet the people walk away.

So, if you’re in a car with airbags, there are a few things you need to do: You need to realize they’re there, all the time. An airbag is the kind of thing that will wait patiently for 20 years or more for it’s big moment, then deploy in under a heartbeat. It’s easy to forget the damn things are there. But you really need to remember – because getting in its way at exactly the wrong time is bad.

The driver’s front airbag is packed into the hub of the steering wheel. So, if you drive one-handed, with your arm across the steering wheel (say, with your right arm at the 10 o’clock position or your left at 2 o’clock) and have a crash while you’re driving that way, your arm will be between the airbag and your head. And while it’s easy to joke about having ‘Seiko’ embossed permanently into your forehead afterwards, if there is an afterwards, the real problem is that your arm will spoil the deployment, and the airbag won’t be able to protect your head. This is just another reason why driving two-handed with hands at 9 and 3 o’clock isn’t optional.

Passengers have responsibilities, too. The passenger’s front airbag (which is bigger than the driver’s because it has to fill a bigger space, and must therefore deploy even faster) comes out of the dashboard. How often have you seen a front-seat passenger driving with a leg crossed over the knee or, worse, a foot (or feet) on the dash? Imagine what happens there – knees blown back into chests at 300km/h (the approximate airbag deployment speed), and no subsequent head protection…




  • Mitch

    Is it true that you nearly always break your nose during a crash with air bags?

    • S

      Not sure, but i’d rather break my nose than my skull.

      • Gary

        The Fellow in the Toyota that ploughed into the car behind me that was pushed into the back of my car unfortunately did not break a nose when his air bag deployed.

    • Wazza

      Never been in a prang so i would’nt know.

  • yonta

    Mitch-
    not if you sit in your seat backwards.

  • Brad

    Interesting article. Thanks.

  • Yonny

    Good article. As I understand it, a large percentage of people who die on our roads weren’t wearing their seatbelts. I imagine that must mean that if they had been wearing belts they may not have died. I don’t know about anyone else, but I literally cannot move my car until I am buckled in.

    I’d like state governments to release detailed breakdowns on the road toll, showing all relevant factors (such as not wearing seatbelts, presence of alcohol/drugs), but I guess this wouldn’t help to maintain the propaganda that speed is the number 1 killer on our roads.

  • Bob

    Yonny, you are spot on!

    Add to that the variable of ‘optional’ maintenance of even newer cars… How often do you see flash cars with bald tyres?

    But no, speed is the number one killer!

  • Peter

    Add to that the further thought that goes into cars like (my fave) Volvo. Essentially, most (if not all) cars have side impact systems of some sort inside the doors. The problem is, though, that bugger SUVs will intrude into the cabin over the window line, which is where most intrusion bars end. Volvo has (at least in the s40, and I’d expect in other models save for the c70) bars that go through to the roof. It makes for a bigger pillar (which can be a little nuisance), but it is a real comfort when your kids are being driven around in one of them. This might be getting more common with other makes, but really I only keep track of volvo and jag, so I wouldnt know. Yeah, they depreciate like nothing else but if they save someone once in your life, it’d be worth it.

  • Lancer black

    Brilliant article. very informative and interesting.

    im thinking ill start driving 9 & 3 from now on.

  • Shak

    I guess people will stop bagging Holden for the monstrous A-pillars in the VE now. because in the event of a roll over (very unlikely), or a side impact it will divert most of the force into the rear and roof pillars. Fair enough it may be a nuisance but it just make you safer both actively (forcing you to look at every turn or roundabout), and passively(saving you from that stupid Soccer mum in her Cayenne)

    • Phill

      The problem with the commodore is those A pillars are what will cause the airbag to go off in the first place.

      • Baddass

        Sorry Shak that isn’t a viable excuse. The Falcon has thinner A-pillars (allowing better visibility and crash-avoidance) and still manages to get a five-star crash rating. Holden just didn’t design them well enough.

    • Andrew M

      Many examples of products getting smaller yet stronger at the same time do exist.
      Its not always a trade off.

      Holdens problem is they focused more on other things.

    • John Cadogan

      Shak, I wrote that story. FYI, 23.4 per cent of road death in Australia occurs in rollover. Rollover, like the rest of the things that can kill you on the road is statistically unlikely (the probability of dying on the road in Oz is about 1600 deaths in 210 billion kilometres, but the probability of dying in a crash (ie once you’re in a car crash) is very high.

  • o

    these articles are great keep up the good work

  • Valet Dabess

    so that’s why our road toll is so high, because safety the technology in our cars suck. maybe if those who had 5 star safety rating might have survived their accidents

  • http://skyline The Salesman

    Can anyone help?

    I heard a rumor about a new more advanced deployment system. Apparently the front seats will weigh you and measure your body’s length every time you sit in the car. Based on the information collected and depending on the speed and angle of the impact the car will measure exactly how much it needs to deploy. It might also work along with load limiters?

    • Peter

      Jaggy has that (active airbags). If you get on to the owner’s manual for the XF, which you get online somewhere, it should tell you how it works. It only activates the passenger airbag when someone is in it, too. Mine is an 08 model, so it’s not too new.

      • Peter

        Sorry, they arent called active airbags, but whatever they are called it has them. I think it also adjusts the tension of the seatbelt too. Strangely enough, they dont publicise this in the blurb, rather it was something I came across in the manual.

        • peter

          Scratch that, I cant find it in the manual, and I might be fibbing. I’m sure I read it somewhere, though.

        • Andrew M

          Do you mean dual stage with crash severity sensors???

          • peter

            it definitely has that, but I read somewhere (I think on the XF Forum) that it also measures weight etc. I know it wont activate unless there is someone in the seat unless you override it, but I cant find in the manual the bit about measuring weight, so maybe that wasnt right.

    • John Cadogan

      I think they are called adaptive airbags, which come in two kinds. The first is a two-stage airbag (small deployment for a small crash; big deployment for a big crash). The second is the kind that weighs the occupant and alters the way it deploys so that it best protects the specific occupant. (Normal airbags are usually calibrated for a range of human masses around average, so you’re especially at risk if you’re above-average in the weight department. John Cadogan

  • Matt

    Good article. You mentioned the bit about having your arms crossed over the wheel, I thought you would’ve also mentioned that the reasons manufacturers say certain things (eg baby seats) shouldn’t be placed in the front seats is because of airbag deployment.

    Having a chat with some other mates that like their cars the other day, the question came up – if the various governments can make things mandatory for a car to be sold in Australia (eg seatbelts, upcoming requirement for stability control, upcoming Euro4 emissions), why don’t they make a requirement that vehicles have to score a minimum (say 4 star) in ANCAP tests to be sold in Australia?

    • Shak

      That would exclude a lot of cars from our market and some entire manufacturers line ups would be cut from Australia. It is just too general. Most utilities and other vans would be culled.

      • Andrew M

        Shak,
        they are bringing in those sort of laws regarding DSC/ESC I think.

        Under those laws commercial vehicles and lowly sold vehicles are exempt.

        Sort of not a bad point what Matt suggests on 4 star min, most are at that point now, and if excluding commercial vehicles and low volume makes was an exemption, it might be a bit more reasonable

        • Shak

          I would agree to that. If they could create a different set of laws for commercial vehicles and normal passenger cars then we would all be a lot safer on the road.

    • John Cadogan

      Excellent comment. Thanks. The Federal Government is dragging the chain badly on minimum crash safety standards for vehicles – John Cadogan (I wrote the story you’re commenting on).

  • Phill

    Never reaily been a fan of the airbag.Theres something about an Expolsive charge going off in your face that breaks your nose and give lascrations on the forearms in the name of safty.

    • Acfsambo

      I would rather a broken nose and lacerations than be dead or a vegetable.

      They should bring in the minimum 4 star rating. The way the should and probably would do is give manufacturers say 2-3 years before they have to have a minimum of 4 stars.

      Also it’s not speed that kills you, it is the sudden stop that kills you. No one has been killed speeding, its when they stop they get killed.

      • Phill

        Acfsambo – I agree that I would rather a broken nose than be dead or a vegetable but there always another side to a story.I googled AIRBAG INJURIES and there are storys of peoples thumbs being ripped off,multiple forearm fractures and snapped wrists,dislocated sholders as well as burns to the face and blindness not to mension death espesally to small people who sit close to the wheel and children.All from relativly low speed crashes,which in most cases the damage to the car is minor.Curtain side air bags are dangerous if someone is leaning on the door when deployed.The funny thing is when i googled airbag injuries,there were alot of law firm link’s

        • Peter

          I think there are a lot more injuries in the US than in Oz, coz ours are designed for use with seatbelts so you dont get the full force of the bag. That said, apparently it scares the crap out of you when they go off and at least the earlier models used to leave burns etc. When airbags first came out, I recall volvo’s line was their cars didnt need them and they didnt make the car appreciably safer, but ultimately they put them in because the perception was that they made the car safer. The reality might have caught up with the perception, I suppose.

        • The Real Car Fanatic

          238 deaths between 1990 and 2002 in the US by the deployment of airbags is not bad. I think you will also find that of these 238 deaths a significant number of them were not wearing seat belts. I guess that why the Airbag is called a Supplemental restraint system.

          Look up supplemental in the dictionary Phill, then you might understand more about airbags and how vital they are.

          • Phill

            Real – look up safety in the dictionary,notice how its not called a supplemental SAFETY system,because it would not be true to the word.

  • Roman

    Great article. Was good to see someone finally mention passengers’ seating position when travelling in a car (ie. not having your feet on the dash etc). Seems to be a fact that is often overlooked in other articles I’ve read on the safety attributes of vehicles.

  • Callous Aussie

    With all this safety technology I am amazed vehicle manufacturers haven’t made seatbelts that are linked to the ignition. No seatbelt , no start. It can’t be that hard.

    • peter

      They had that in some cars in the 80′s, from memory, and it drove everyone mad. I can remember Paul Hogan doing a skit on it. The “trick” was to put the passenger seat belt into the drivers seat fastener. Dont blame me, I was only a youngster…

  • xBeanie

    I think there is some debate around what is “passive” and what is “active”. To my way of thinking “active” means it does something explicitly to prevent or minimise the impact of an accident and whether it is triggered by the driver, computer or mechanical device is irrelevant. I would see features such as crumple zones, anti-submarining seats and good quality tyres as being passive safety features.

    Excellent article, nevertheless.

    Some of the comments were predictable. There is a lot of misinformation out there on this topic and I love how some people are eager to believe it. Of course its all just a government conspiracy to keep the seatbelt and airbag people in business.

    • Camski

      xBeanie – You know those ‘active’ pre-coll. systems?
      I think that helps in the understanding of passive and active. Your definition of active appears to be correct to me, however for it to be active, I believe the “system”, whichever it may be, needs to be able to vary itself in some way.

      For example, the pre-collision systems vary many aspects of the car, where as a seat belt is either on, or off.

      This means that in some aspects, the newer low/high impact airbag systems could be classified as active because they are no longer just deploy or not-deploy, but have a variable that affects how the deploy occurs.

  • Davo

    Another factor that comes into play is speed at impact.
    I don’t doubt that Airbags can be a life saver.
    Not long ago survival in a 60kmh collision was seen as being difficult to achieve even by the best in the industry despite crumple zones & efforts to control the energy away from the driver & occupants.Much was made of this in sale pitches of numerous makes.
    My suspicion is once a certain speed is reached nothing will save you no matter how many airbags are deployed.
    PHOTO of the jeep & lighter wagon used before but the structure of the JEEP is buckled & bent whereas the other is still largely intact.
    Being Airbag equipped is only part of the equation.

  • LessQQ

    I can think of one.

    They can introduce ejection systems for high speed collisions like in jets.

    LOL