You know how time seems to slow down to almost a dead stop when something unexpected happens? Well, that ain’t nothin’, pal.
This post at a blog called Sentient Developments compares the amount of time it takes for the various on-board anti-crash technologies in your car to react to and counteract a collision to the amount of time it takes the human brain to register that it is about to be involved in an accident.
Here’s the timeline of events, measured in milliseconds:
0 milliseconds – An external object touches the driver’s door.
1 ms – The car’s door pressure sensor detects a pressure wave.
2 ms – An acceleration sensor in the C-pillar behind the rear door also detects a crash event.
2.5 ms – A sensor in the car’s centre detects crash vibrations.
5 ms – Car’s crash computer checks for insignificant crash events, such as a shopping trolley impact or incidental contact. It is still working out the severity of the crash. Door intrusion structure begins to absorb energy.
6.5 ms – Door pressure sensor registers peak pressures.
7 ms – Crash computer confirms a serious crash and calculates its actions.
8 ms – Computer sends a “fire” signal to side airbag. Meanwhile, B-pillar begins to crumple inwards and energy begins to transfer into cross-car load path beneath the occupant. 8.5 ms – Side airbag system fires.
15 ms – Roof begins to absorb part of the impact. Airbag bursts through seat foam and begins to fill.
17 ms – Cross-car load path and structure under rear seat reach maximum load. Airbag covers occupant’s chest and begins to push the shoulder away from impact zone.
20 ms – Door and B-pillar begin to push on front seat. Airbag begins to push occupant’s chest away from the impact.
27 ms – Impact velocity has halved from 50 km/h to 23.5 km/h. A “pusher block” in the seat moves occupant’s pelvis away from impact zone. Airbag starts controlled deflation.
30 ms – The Falcon has absorbed all crash energy. Airbag remains in place. For a brief moment, occupant experiences maximum force equal to 12 times the force of gravity.
45 ms – Occupant and airbag move together with deforming side structure.
50 ms – Crash computer unlocks car’s doors. Passenger safety cell begins to rebound, pushing doors away from occupant.
70 ms – Airbag continues to deflate. Occupant moves back towards middle of car. Engineers classify crash as “complete”.
150-300 ms – Occupant becomes aware of collision. (emphasis mine – BK)
There is some debate as to that last mark, with some saying it can take as long as half a second (500 milliseconds) for the human brain to recognize what is happening. As some of the commenters there point out, though, your brain is likely to infer that an impact is imminent from other cues (namely seeing whatever you’re about to crash with coming straight at you) long before your brain begins to process the input from experiencing the crash. In the original example mentioned in the post, though (being shot in the head), a bullet does indeed move too fast for the human brain to perceive as it moves toward you. So maybe you’ll get lucky and be killed by a bullet someday!
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