Metacognitive judgments of alcohol-related intoxication and impairment: The Mellanby effect and alcohol myopia in low-risk drinkers

• Drivers are more confident in their abilities to drink drive prior to drinking. • Low-BAC groups overestimated and high-BAC groups underestimated intoxication. • Drivers may feel less impaired on the descending limb of intoxication. • Driving decisions rely more on BAC estimates than perceived impairment. • High confidence in BAC estimates significantly increases the risk of drink driving. Introduction: The decision to drive after alcohol consumption is largely influenced by an individual’s interpretation of their alcohol intoxication and level of driving impairment. However, such perceptions may be influenced by the level and duration of intoxication. This study investigated the effect of alcohol intoxication on drivers’ judgments of their impairment and driving capabilities across different blood alcohol concentration (BAC) levels. Method: One hundred and sixty blinded participants were randomly allocated into low (∼.025%), mid (∼.055%), or high (∼.085%) BAC condition groups and completed surveys about their perceived alcohol intoxication and driving capabilities at various time points. Results: The results showed that the mid BAC condition group was most accurate in estimating the number of standard drinks consumed, whereas the low condition group overestimated, and the high condition group underestimated their alcohol consumption. While participants overestimated their BAC, they became more accurate when their BAC was falling. Between-group comparisons showed that participants rated their driving capabilities lower in the higher BAC condition groups. However, prior to consuming alcohol, the mid and high condition groups were significantly more confident in their ability to drive over the legal limit. Despite having a higher average actual BAC, the high condition group reported greater driving capabilities when their BAC was falling and a lower estimated BAC compared to the mid condition group when BAC was rising. Finally, logistic regression analysis revealed that estimated BAC and estimation confidence, rather than perceived driving impairment, significantly predicted the decision to drive after drinking. Conclusions: These findings suggest that the level and duration of alcohol intoxication may influence a driver’s perception of impairment and their subsequent decision to drive.