Once a person has been drinking, they underestimate the risks of being on the road, and their ability to tolerate alcohol.

Aah, studying for your license? No worries.
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Imagine you are a drinker. Just imagine, even if you're under 21. You're drunk. You're gonna go home. Smoke 20 packs of alcohol. Drink some cigarattes. Sleep on the floor, and stand on your bed. Everything is swimming around you
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As you can see, being a drinker makes your brain fuzzy and confused. It makes the nerves and reactors in your brain slow down. It also makes you respond and react to things slower. This, all in turn, makes you drunk. When you are drunk, you don't remember certain things and make absurd decisions. Your decisions are usually not very smart. So, when you drink and then decide to drive, you overestimate your ability to tolerate alcohol.
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Hope I helped!!