Vehicle safety systems have come a long way in the last few decades. Almost all cars come with an abundance of acronyms and unpronounceable terms, but what do they actually do? We unpack some of the most common safety terms and how they work.
I often get asked: “What is the safest car on the road?” I usually say that I can only answer that question if you can tell me exactly what kind of accident you’re going to have. This illustrates the fact that no two accidents are identical and that each automotive company concentrates on their own interpretation of accident statistics from various countries to guide them in developing safety items.
These items are usually classified as either active or passive. Crumple zones, seat belts and airbags are passive safety items that try to reduce the severity of an accident’s aftermath. ABS (anti-lock braking system), traction control and ESP (electronic stability programme) are active safety devices that make it easier for drivers to avoid an accident.
Passive safety items
What happens when a car hits a large object? The car comes to a very quick stop or slows down very fast. The passengers carry on moving for a fraction of a millisecond until they hit the dashboard, windscreen or steering wheel at the original speed. Seatbelts were introduced in the late 50s as the first line of defence against the subsequent injuries. Initially, lap belts and later lap and shoulder belts held occupants' bodies to a selectable tension. Later, inertial belts allowed slow body movement, but tightened up when a car decelerated at an alarming rate.
On some of the latest cars, radar sensors measure the direction, speed and size of a nearby vehicle, and warn a control unit when an accident is imminent so that seatbelts and airbags can deploy rapidly. Another innovation monitors the severity of the braking application and will tighten the occupant’s seatbelts, and move the seat away from the fascia if it senses that the driver is braking very fast. If the car skids the system closes the side windows and the sunroof because these closures prevent the occupants being thrown out during an impact.
You can still find people who are under the impression that a car with a strong and rigid framework is safer to have an accident in than a less rigid car. This perception was challenged by the Hungarian Mercedes-Benz engineer Béla Barényi (1907—1997), who is credited with inventing crumple zones, the non-deformable passenger cell, collapsible steering columns and many other safety devices.
Barényi realised that a rigid structure transmits the shock load that accompanies an accident almost immediately to the passengers, often in undiminished form. In contrast, a correctly-designed deformable structure will crumble progressively. This will not only dissipate energy, resulting in reduced shock loads but also slow down the transfer of these loads to the people inside the car. In many types of accidents, this slowing-down process reduces the deceleration felt by the passengers from a more than a fatal 100g to less than 30g. (g = the acceleration due to gravity)
On its own, this feature will help to save lives, but when it is used in combination with airbags and seat belts the safety levels increase dramatically. Practically all modern cars have crumple zones, but very few commercial vehicles have reached this stage of development.
The first airbags deployed when a single accelerometer on the car showed that the car had reached a level of deceleration that usually denotes an accident. Modern airbag controls are more sophisticated. In the event of a collision, a number of strategically-placed accelerometers will send signals to the processing unit, which the latter will interpret as either a soft collision, meaning no deployment, or a hard collision, which requires deployment. This decision and the time to signal the airbag takes between 15 and 30 milliseconds. The airbag then deploys in about 30 milliseconds. This is usually a short enough time to protect the passengers from the worst effects of the shock.
Modern airbags don’t deploy unless the seatbelt is in use. The bag comes out at over 300 kph and can injure or kill small-bodied people. This is why drivers are warned not to strap small children into a front seat, but rather employ a specially-designed child seat.
Active safety items
The presence of a computer makes it possible to fit modern cars with active safety items. Here are some of the most noteworthy:
ABS (anti-lock braking system)
ABS is been designed to make it easy to keep the car in a straight line during panic braking. This system employs a speed sensor at each wheel and whenever the controlling processor senses that one wheel shows a sharp reduction in speed compared to the others it enables a pump to pulse the pressure in the brake fluid line going to that particular wheel. The result will be that the driver can brake as hard as they like, but the wheels will not lock up, nor will the car veer to one side.
Employs a speed sensor inside the brake servo to sense when the driver is braking really fast. The system then activates the brakes to the maximum extent. This is necessary because many people panic to such an extent during severe braking that they do not apply full pressure on the brake pedal.
Employs the ABS wheel speed sensors to inform the processor whenever one wheel rotates significantly faster than the others. It will then cause that wheel to be braked until it’s travelling at the same speed as the others.
ESP (electronic stability programme)
Stabilises a car’s movement during cornering by braking the left rear wheel to correct understeer (car tends to go straight) or the right front wheel to correct oversteer (rear wheels break away). The controlling processor employs the ABS wheel sensors, a yaw sensor (yaw = rotation about a vertical line through the centre of gravity), plus a steering wheel rotation angle sensor to determine whether a car is cornering normally or the driver is on the point of losing control.
Adaptive cruise control
Employs front-mounted radar or laser sensors plus a video camera to maintain the driver’s chosen speed and also brake if the car in front slows down. It then accelerates back to the chosen speed when the car in front speeds up. Older systems will start the braking procedure but warn the driver to take over. Some of the later systems will brake to a standstill if the other car stops.
Rearward-facing cameras, adaptive headlamps and night view assist
Helps the driver to see better, especially when it’s dark. Lane centring uses cameras to continuously monitor a car’s position on the road in relation to side- and centre markers. If the vehicle wanders off to either side without the activation of a turn signal, the driver is warned by a buzzer or a vibrating steering wheel. In this way, the dangers inherent in intoxication and sleepiness can be reduced.
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