In the United States we tend to think of drunk driving as being the same thing as a DUI. A DUI is not AT ALL the same thing as drunk driving. A DUI is the criminal offense resulting from driving while particularly drunk. But currently the criterion for being charged with a DUI is that you are driving with a blood alcohol content (BAC) of 0.08% or higher.
There is no scientific evidence of a threshold BAC below which impairment does not occur.1. BACs of as low as 0.02% have been shown to cause difficulty with tasks that demand divided attention (such as driving). In 2000, approximately 16% of all deaths due to drunk driving were caused by people with BACs of less than 0.08%2. That is to say, 1 out of 8 people killed by drunk drivers were killed by people who the police would not charge with drunk driving, people who had had only 2-3 drinks and who did not feel drunk, and people who legally were not drunk.
Deaths, of course, are only the most dramatic consequence of drunk driving. These same legally sober drivers caused 43,730 accidents (again, in 2000), causing an estimated 14 billion dollars worth of damage.
You cannot judge how impaired you are after drinking. It will depend on your base mental state, the number of drinks you have had, the type of drink you had, the time since you had them, the amount of food in your stomach before drinking, and other factors that you are totally unqualified to juggle even when sober. You cannot judge how impaired you are after drinking, but you can take a cab.
Don't drink and drive.
At all.
References:
1. Ogden, E. J. D. and Moskowitz, H. (2004). Effects of Alcohol and Other Drugs on Driver Performance. Journal of Crash Prevention and Injury Control, 5(3), 185-198.
2. http://www.nhtsa.gov/people/injury/alcohol/impaired_driving_pg2/US.htm
If you can find more recent data, please let me know.
And the secondary source and inspiration for this writeup, freakonomics.blogs.