Everybody knows about the dangers of driving under the influence, but still there are thousands of car wrecks each year related to alcohol use. Few people drive drunk on purpose--instead, many of these are accidents stemming from poor judgment, or judgment clouded by alcohol. If you don't have a designated driver and you've been drinking, you need to have a way of knowing, with absolute certainty, that you are sober enough to drive.

Many people rely on personal rules of thumb to ensure their sobriety on a night out. Whether their trick involves only having a certain number of drinks, drinking a glass of water for every drink, or other methods, these tricks can be deceptive. The fact of the matter is that alcohol can affect your body differently every night--so you can never be sure your method will work the same way twice.

Your level of "drunkenness" depends on more than just the amount of alcohol in your system. It will also depend on what you've eaten recently, how hydrated you are, if you've been sick or missed sleep recently, and so forth. Three beers could leave you stone sober one night and cause you to be too drunk to drive home the next--so be careful.