Alcohol impairs judgement and thought processes. Since judgement is impaired, a person may take more driving risks.
Intoxicated drivers tend to underestimate the speed of their autos.
Alcohol also slows a person's reflexes. Alcohol makes the pupil of the eye slower to respond to changes in light. Oncoming headlights are thus more likely to blind an intoxicated driver. Gilman AG, Rall TW, Nies AS, Taylor P, eds. The pharmacological basis of therapeutics, 8th ed.