the standard establishes a
performance test to ensure that helmets
will adequately protect the head in a
collision.
...
Consistent
with the requirements of the ANSI,
Snell, and ASTM standards, the peak
headform acceleration of any impact
shall not exceed 300 g for an adult
helmet...

All car designs sold in the U.S. must be tested on the G-forces registering on a safety-belted, front-seat dummy when its car crashes into an immovable barrier at 30 mph. To pass the test, the dummy’s chest deceleration cannot exceed 60 Gs for more than three milliseconds during the crash, according to the Federal Motor Vehicle Safety Standard 208.

Minimum force levels associated with face and skull fractures caused by blows with a small impactor (6.45 cm2 area) range from 670 to 4000 N (Schneider and Nahum, 1972). These forces correspond to head accelerations of 15-90 g for the 4.5 kg head of the Hybrid III dummy used in Institute tests. This additional head evaluation criterion is similar to requirements of the Canadian Motor Vehicle Safety Regulations Standard 208, which limits test dummy head accelerations to less than 80 g in cars not equipped with airbags.