Drunk driving, you've always heard, happens when you go over a blood alcohol concentration of 0.08%. That's the legal limit. If you drink more than that and drive, you're breaking the law.
This is not entirely accurate. Yes, exceeding 0.08% and driving is illegal. However, what the law really prohibits is impaired driving. You can be impaired when you're under 0.08%. If you blow a 0.07%, you could get a DUI.
How does it actually work? The police have to show that the alcohol impaired your ability to drive. If you're over 0.08%, they don't need any more proof than that. The law accepts that you were impaired with a BAC that is that high.
If you're under 0.07%, then they usually need to present more data or evidence to demonstrate that you were impaired. Examples could include the fact that you caused a car accident or an officer who watched you swerving all over the road and breaking traffic laws.
You also have to take into account that BAC levels change over time. Maybe the officer couldn't give you a test on the side of the road. You didn't get tested until two hours later, and then a blood test showed a BAC of 0.07%. They may argue that your BAC would have fallen by more than 0.01% in two hours, so you must have been over 0.08% when you were behind the wheel.
As you can see, these cases can get complicated. It is important not to buy into the common misconceptions and to really understand what legal defense options you have.