The sad fact is that it is actually pretty hard for a cop to catch a drunk driver. Everybody knows that it doesn't take very much alcohol to impair a driver however most people when they are legally drunk don't drive like an epileptic monkey. Some people in fact appear to drive better. That is because they concentrate very hard on what they are doing and keeping under the speed limit and so on. However the same person who has their full concentration on keeping it rubber side down is also not able to keep aware of what is going on around them and if the unexpected happens, they will not have the reaction time act accordingly. So how do you know to pull someone over if they appear to be driving perfectly. There is also the ratio of drunks to cops. For every drunk who is stopped there are probably 25 more who are not. Think about how many people empty out of valley bars every night between the hours of midnight and two.