In a car crash involving a modern vehicle, everything happens before the occupant is even aware of the collision.
1 ms - The car's door pressure sensor detects a pressure wave.
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.
20 ms - Door and B-pillar begin to push on front seat. Airbag begins to push occupant's chest away from the impact.
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.
(via gulfstream)