Alcohol affects the brain, reducing a person's ability to think clearly. The driver may not and actually feel he / she is performing better than they actually are.

There are two ways to view intelligence or ability:
ability it is fixed or ingrained – in other words, we are born with a certain level of ability and we cannot change that. this is called a fixed mindset.
we can develop our ability through hard work and effort. this is called a growth mindset.
these two different beliefs lead to different behaviour, and also to different results. for example, students with a growth mindset were shown to increase their grades over time. those who believed that their intelligence was ingrained did not; in fact, their grades got worse.
having a growth mindset (the belief that you are in control of your own ability, and can learn and improve) is the key to success.
yes, hard work, effort, and persistence are all important, but not as important as having that underlying belief that you are in control of your own destiny.