Momentum = (mass) (speed)
The driver's initial momentum is (125 kg) (31 m/s) = 3,875 kg-m/s
At rest, his momentum is zero.
The change in his momentum is (0 - 3,875 kg-m/s) = - 3,875 kg-m/s .
The average force exerted on him by the airbag is
(3,875 kg-m/s) / (1.5 s)
= 2,583.333... kg-m/s²
= 2,583.333... newtons (about about 581 pounds)
That's what it takes to stop a 125 kg mass moving at 31m/s in 1.5sec.
The negative acceleration is 31/1.5 m/s² = about
= 20.333... m/s² = about 2.1 'G's.
If he were not wearing his seat belt and the airbag did not deploy,
then he would not be brought to rest ... at least not until he hit the
windshield at 31 m/s ... about 69.3 mph .