People make the decision to drink and drive every day. It's not reserved just for teenagers. People that drink and drive never believe that they've had too much to drink; they don't think their reflexes are any slower because they've been drinking. Usually, it takes something tragic to happen before they change their behavior.
My friends and I always had a designated driver, and my daughters know that I will pick them and their friends up no questions asked, if they find themselves in a situation where the designated driver screwed up.
In college, my future husband and I had two drinks at a comedy club. We were NOT drunk, but we were driving down this dark street and there were these two rectangular lights moving up and down, up and down. We were like what the heck is that?!? So we slowed a fraction (not nearly enough), and continued to drive forward, watching the lights move up and down, up and down. We couldn't figure out what they were, and then we were almost on top of them.
We almost hit a dumb bike rider who was wearing all black. All we had been able to see were the pedals. We took turns being the driver after that. It was about the most horrifying experience in my life, and it was two drinks with a meal.
that's a good question. i believe its because they dont realize theyre drunk, they think theyre just fine. ive never lost any friends or relatives due to drunk driving, just tired driving, so idk for sure...