Question

When the car carrying the unbelted driver crashes, what brings the driver to a stop

Answer 1

When the driver is not wearing a seat belt and the car hits a brick wall,
the driver keeps going and doesn't stop, (because of the inertia of the
driver's body). 

-- If the driver is lucky, the car is going slow enough, AND it has a
driver-side airbag, AND the airbag works, AND it inflates fast enough,
AND it cushions the driver well enough to avoid the driver's death.

-- If an airbag doesn't stop the driver, then one or more of these brings
the driver to a stop:

... the car's engine block crashing in through the dashboard 
... the steering wheel
... the windshield
... the same brick wall that stopped the car.

Any one of these is probably fatal.