Generally, states use BAC to prove impairment. Often, if the BAC is above a threshold value (depends on the state), impairment is assumed. If BAC is below that

Generally, states use BAC to prove impairment. Often, if the BAC is above a threshold value (depends on the state), impairment is assumed. If BAC is below that threshold, the BAC is evidence of drinking, but not impairment. Then other evidence, like officer/witness testimony, crashing the car, speeding, etc., are brought in to prove impairment.