After drinking, the brain works inefficiently, taking longer to receive messages from the eye; processing information becomes more difficult and instructions to the muscles are delayed. Alcohol can slow down reaction time by 10 to 30 per cent.If this is a reason to ban drink driving then many over 30s should not be allowed to drive. A research paper entitled "Simple reaction time, duration of driving and sleep deprivation in young versus old automobile drivers" reports that the median reaction time for drivers under 30 was 236ms. If that is increased by 30% we have 306.8ms.
In other words, the law currently assumes that someone whose reaction times are over 306.8ms should not be driving. The problem is that the median reaction time for over 30s is 262ms with a standard deviation of 81ms (the slowest recorded was 448ms). This means that quite a lot of drivers over the age of 30 are driving with reaction times equivalent to being drunk while driving.
Obviously I'm being facetious. But the point is that the law is far too arbitrary. As it stands people are being punished without any evidence that they (as individuals not statistics) are incapable of driving properly.