Death-defying designs for car safety
This topic is sponsored by NRMA ACT Road Safety Trust.
Car crumple zones and airbags designed to absorb crash energy are contributing to a lower road toll.
Media reports might make you think that car and car parts manufacturers are a road user’s worst enemy. For example, a leading manufacturer in the United States has voluntarily recalled millions of its tyres because of possible safety risks, and a major car-maker in Japan has admitted misleading the government and the ordinary citizen for decades over customer complaints about defects that could have cost lives.
Such reports shouldn’t mask the fact that our roads are safer now than since the earliest days of the automobile (Box 1: Declining road toll). This welcome turnaround is due to a number of factors, including an improved road system, which makes it easier to drive safely, and random breath-testing, which helps keep drunk drivers off the road. A third factor despite recent bungles is an increase in car safety.
Cars have become safer partly because they are now stronger where it counts. To protect occupants from a direct impact, the passenger compartment should keep its shape in a crash. The Australian Design Rules, which set out design standards for vehicle safety (and emissions), specify such things as strength and stiffness requirements for side-doors to help protect occupants in a side impact; collapsible steering columns to minimise the risk of crushing or piercing injuries from a frontal impact; anti-burst door locks; and padding on instrument panels.
But ‘weakness’ can also be a strength. The fronts of many cars are now manufactured with ‘crumple zones’ designed to absorb as much of the crash energy as possible in frontal and rear impacts. In effect, crumple zones act like the cardboard boxes used in movie stunts to break a fall. As the stuntman falls he gains kinetic energy; if he was to hit solid ground, most of that energy would be absorbed by his body in a very short time, with possibly fatal consequences. However, a well-designed stunt will ensure that the stuntman lands on a stack of cardboard boxes; the impact energy will then be used to collapse the boxes instead of to damage the stuntman. Similarly, the purpose of car crumple zones is to minimise the amount of crash energy transferred to passengers when a vehicle collides with a solid object.
The heroic seatbelt
There are other reasons for the increase in car safety. It is no coincidence that the decline in Australian road deaths commenced in earnest in the years following the introduction by 1973 of laws in all Australian states and territories making it compulsory to wear seatbelts. In fact, car accident researchers in Australia estimate that seatbelts reduce the risk of fatal injury to front-seat occupants by 45 per cent and the risk of serious injury by 50 per cent. The United States National Highway Safety Administration reports that 3 out of every 5 people killed in vehicle accidents in the US where seatbelt use is much lower than in Australia would have survived their injuries had they been strapped in.
Why seatbelts are effective
Stopping suddenly imposes a great deal of force on all objects in the vehicle. What the seatbelt does is distribute that force to the some of the strongest parts of the human anatomy the chest and pelvis. Crash survivors will often have seatbelt-induced ‘burns’ and bruises in these areas although this is far preferable to concentrating such forces on the head or at a puncture point in the chest or abdomen. Without a seatbelt, the occupant will continue to move forward in a frontal impact until brought to a stop by an object such as a windscreen, steering wheel, dashboard or front seat. In the most serious accidents, the seatbelt may cause internal injuries, while unrestrained occupants will probably be killed instantly.
Seatbelts play other safety roles. Unbuckled occupants become missiles that cause injury to other occupants should they collide with them. Being thrown from the car is usually equally calamitous; in a rollover, an unrestrained occupant who is thrown out of the car is likely to be crushed. In a frontal impact, occupants propelled from the car will be injured both by passage through the windscreen and on impact with the ground or other solid object.
Airbags are a more recent addition to the armoury against road trauma. Most commonly, these are located in the centre of the steering wheel and above the glovebox on the passenger side. They are designed to activate almost instantaneously on impact to form a cushion as the head and chest of the driver or passenger flex forward. According to the Australian Government Department of Transport and Regional Services, this is what happens when an airbag deploys in a crash:
- In the 15 to 20 milliseconds after impact, the crash sensors and control unit determine the severity of the collision and decide whether to deploy the airbag.
- At about 25 milliseconds, the airbag splits its covering pad in predetermined places and begins to inflate rapidly.
- At about 45 milliseconds, the bag is fully inflated while the seatbelted occupant is still moving forward.
- At around 60 milliseconds, the occupant contacts the airbag, which immediately begins to deflate via vent holes in the back.
- Up to 100 milliseconds, the occupant continues to sink deeply into the airbag, which cushions the head and chest while it is deflating.
The airbag therefore functions in much the same way as the stuntman’s cardboard boxes. It also acts as a ‘friendly’ barrier between the occupant and the hard, less forgiving surfaces of the car interior.
Like the seatbelt, airbags are widely credited with saving lives US statistics suggest that the risk of fatality in a frontal impact is reduced by about 30 per cent by the deployment of an airbag. Crash tests in Australia indicate that the risk of serious head injury is reduced by 50 per cent or more in most popular makes of family-sized car.
An airbag is designed to be fully inflated by the time the occupant’s head makes contact with it. Anything else would be dangerous: a collision between a head, moving at high velocity in one direction, and the bag, moving at a similar speed in the other, could be fatal. And accidents do happen airbags have been implicated in some deaths in the United States. Since car occupants there are less inclined to wear seatbelts, the airbags are designed to inflate more quickly and thus with more force than those used in Australia. Australian researchers have found no evidence of death or injury caused by airbag deployment in over 100 investigated cases.
Airbags can be dangerous to children and small adults even when they are restrained by adult seatbelts if they sit too close to the airbag or are not seated correctly when the airbag inflates . Moreover, rearward facing ‘cradle’ or capsule-type infant restraints should never be placed in a front passenger seat that is fitted with an airbag. In the event of a crash, the baby's head would be within the ‘strike’ zone of the airbag and the impact could lead to fatal head injuries. All occupants of cars with fitted airbags need to be aware of the potential danger and ensure they are seated correctly. In Australia, children and infants are more often buckled into the rear seat, which is much safer.
Despite the potential hazard, airbags have proved both effective and popular. Some makes and models of cars now have side airbags to help prevent injury to the head and shoulders caused by side impacts which account for about one-quarter of all crash injuries and 28 per cent of deaths.
The safety features discussed so far are designed to protect car occupants in the event of a crash. The purpose of an anti-lock braking system (ABS), on the other hand, is to avoid the crash in the first place. It helps do this partly by reducing stopping distances; in other words, ABS will stop a car more quickly than will conventional brakes. As the name implies, ABS is designed to stop brakes from ‘locking’ which is when the wheels stop rotating and the tyres start skidding. By preventing locking, ABS reduces the loss of traction in a stopping emergency and increases the driver's ability to steer the car.
The system works simply enough. Sensors located on each wheel are monitored by a computer called a controller. Just before the wheel locks it will experience a rapid deceleration, or loss of speed. This is noted by the controller, which opens a valve to release pressure on the brake, thus allowing the wheel to keep moving. The pressure is then quickly reapplied (otherwise the car wouldn’t be brought to a stop) but eased whenever the wheel is about to lock. In this way, the wheel is brought to a stop without ever having locked. The driver applying the brakes in an emergency will feel a pulsing in the brake pedal, which is the result of the alternate increasing and decreasing of pressure on the brakes by the controller.
Expect more safety gains
The strategic strengths and weaknesses that manufacturers are now building into their cars, along with safety devices such as the seatbelt, airbag and ABS, are the result of scientific and technological innovation, as well as considerable investment by car-makers. They are making cars safer and we can look forward to more improvements in the future. For example, 'smart' airbags and seatbelts that adjust the rate of inflation of the bag or tension on the belt according to the weight of the person are becoming available, and Volvo has recently developed a seat designed to reduce whiplash in rear-end collisions.
But safety devices do not offer a foolproof guarantee: drivers should be responsible for monitoring the safety of their cars and their behaviour behind the wheel.
Posted December 2000.