Typical speeds to use in estimates are walking 1.5 m/s, running 3 m/s, cycling 6 m/s, a car in a town at 30 mph which is about 13 m/s, and a car on a motorway at about 30 m/s. Normal driving gives small accelerations, about 2 to 3 m/s2 when speeding up and about 8 m/s2 in an emergency stop. Free fall is 10 m/s2, so an emergency stop is a little under 1 g.
In a crash the speed drops to zero in a very short time. Acceleration = change in speed ÷ time taken. A car at 15 m/s stopping in 0.1 s has a deceleration of 15 ÷ 0.1 = 150 m/s2, about 15 times the acceleration of free fall. A car at 13 m/s stopping in the same time has a deceleration of about 130 m/s2.
Then use force = mass × acceleration. An 80 kg driver decelerating at 150 m/s2 experiences 80 × 150 = 12000 N. Their weight is only 800 N, so the force is about 15 times their weight, which is why large accelerations in crashes cause serious injury. Compare this with the 8 m/s2 of a normal emergency stop.