Imagine this scenario – you go to the pub after work and when there decide "just to have the one" knowing you will need to drive home. You then think it might be best to leave the car but reconsider saying to yourself "I feel fine " and get behind the wheel .
Or another, you are at home and have a glass of wine or two after a long week. You have no plans to drive until a text comes in asking you to pick up one of your family from the train station. You hesitate for a moment and then say "Actually I feel fine "and set off on your journey.