Drinking and driving reduces your ability to react to things on the road around you. This is just one reason why it is so dangerous. A lag in reaction time could mean you don't stop at a red light or you don't see that traffic slowed down ahead of you until the moment of impact.
When you're driving, one of the things you need to be cautious about is recognizing motorcyclists on the roads. Because of the size of their vehicles, they often appear to be moving more slowly than they are. This tends to result in crashes.