2. Poor Decision Making
Drinking excessive amounts of alcohol before getting behind the wheel can greatly affect your sense of judgment, as well as your depth perception and vital motor skills necessary to safely operate a motor vehicle. You may not realize how close other objects and vehicles are to you, and may be unable to judge speed and distances properly. Even your perspective of space narrows when impaired.
3. Affect Ability to Multi-Task
Driving while drunk can take away your ability to concentrate on steering the wheel properly, stay within the speed limit, and keep an eye on your surroundings at the same time. When impaired by alcohol, you are less likely to notice traffic signs, pedestrians, bikers, or vehicles coming from the opposite side. You may also end up with tunnel vision, and tend to look more straightforward and fail to look to your left or right.
4. False Sense of Confidence
Alcohol can make you feel more confident than you normally would be, letting you take risks that you would not usually take. Being overconfident could lead to reckless and dangerous driving, which is overly dangerous as drivers are not aware of how deteriorated their driving skills have become after drinking. Unfortunately, having alcohol in your system will also leave you less able to deal with unexpected events properly.
Source: Drunk Driving Research, published on http://responsibility.org/drunk-driving/drunk-driving-research.