Information in this table shows the blood alcohol concentration (BAC) level at which the effect usually is first observed. What are the effects of blood alcohol concentration (BAC)? These numbers were summed to obtain the annual national estimates (see ). Note: The annual estimated alcohol-impaired driving episodes were calculated using BRFSS respondents’ answers to this question: “During the past 30 days, how many times have you driven when you’ve had perhaps too much to drink?” Annual estimates per respondent were calculated by multiplying the reported episodes during the preceding 30 days by 12. Behavioral Risk Factor Surveillance System (BRFSS), 1993–2014. 4Īnnual Self-reported Alcohol-impaired Driving Episodes among U.S.