There is no such thing as "absolute safety". If applying a countermeasure to an identified risk requires adding more switches, sensors, wiring, devices, logic, and so forth, then each of those additional elements adds their own elements of risks or failures, and sometimes the entire concept of the countermeasure itself can lead to risks ... the old "unintended consequences".
Let's s'pose that one were to install the big red button that switches off everything in the car, hard-wired, with no computer intervention possible.
What happens if someone accidentally presses it while travelling at 130 km/h on the motorway? What if a passenger accidentally hits it? If you put it in a place where the passenger can't reach it, what happens if a situation arises where the passenger really is warranted in shutting the car down but can't reach it? There IS NO perfect solution because what might be "safer" given one set of circumstances either doesn't solve the problem or makes matters worse given a different set of circumstances.
If you want to get involved in FMEA, what if the wiring to the switch fails? What if the contacts remain open? What if the contacts weld closed? What if the wire shorts-to-B+ or shorts-to-ground or open-circuits? YES these are already existing line items for all sorts of existing components in the vehicle ... you've just added more, by adding another switch and more circuits and more relays and more wiring.
What, exactly, do you have that big red button switch off? If you switch off EVERYthing, you just switched off power steering (it's generally electrically operated nowadays - and even if not, you just switched off the engine and therefore the power steering pump), and you just switched off the power to the crash sensors and the airbag controller.
Hmmm, that sounds a lot like the problem GM had with the Chevrolet Cobalt ignition switches. No big red button, but that was a traditional old-skool hard-wired ignition switch that, when turned off, turned off everything in the car including the airbag module. GM's problem was that the location and orientation of the switch and the force required to actuate it made it prone to being accidentally bumped and switched off while in motion.
Okay, so let's maybe not switch off the airbag module. Does that mean the airbag system has to stay active when the car is switched off? What happens if I get hit while stopped in my car and I've stopped the engine because the traffic has stopped - should the airbag system stay on? If yes - how long? Is the airbag system to remain in wait while the car is parked for three weeks at an airport parking lot?
If the driver switches off the ignition switch while the car is in motion, it's pretty easy nowadays (with electronic controls in the form of a suitably programmed "body control module") to arrange for ABS and electric power steering and lighting circuits and airbags etc to remain powered up until there is suitable evidence via other sensor signals that these systems are not needed.
FMEA is a real thing. The auto manufacturers do it. They are not perfect nor is anyone else, and you can identify situations in which there is no perfect solution but you have to weigh the probability of event X and its consequences against the probability of event Y and that event's consequences.
No, a '63 Ford Falcon didn't have a lot of these things. It didn't have ABS or airbags or electric power steering. You didn't have to worry about when to switch something on or off if it didn't exist in the first place.
In terms of safety I will take my odds in a crash in my modern small car over what would happen in a '63 Falcon ANY day. It may not be perfect ... nothing is ... but we are in a whole lot better position now than we were then, in terms of occupant protection.
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