Originally Posted by Mark in Idaho

No, the braking action of a car is limited to about 1.5 G's. That will not cause a concussion. If you weight 150 pounds, your weight against the seat belt will max out at 225 pounds. Your 8 pound head will feel like it weighs 12 pounds. Laying your head on a pillow carefully is 8 pounds (1 G). Plopping your head on a pillow can easily be 4 or 5 G's or more. Subconcussive impacts generally start at 20 G's for very sensitive brains and 40 to 50 G's or more for normal brains.

If you are caught off guard and your head whipped forward until your seat belt stopped your forward movement, you might experience a few more G's due to the whipping effect. But, if you resisted the braking action, the G force would not exceed 1.5.