To be charged with a DUI, a driver has to be shown to be under the influence and impaired in his or her ability to drive. Evidence that the driver is intoxicated could include video of the driver going out the proper lane, for example.
However, per se DUI laws make it so that the driver can be charged based on blood alcohol concentration alone. When the BAC exceeds the legal limit, the authorities don't have to do anything else to show that the driver is impaired.
What this means is that someone could look like he or she is driving perfectly, showing no signs of impairment, and still be given a DUI. For example, if you are pulled over because you have a headlight out and the police smell alcohol and give you a breath test, blowing over the legal limit means you can get a DUI even if you made no mistakes and broke no traffic laws.
In Nevada, the legal limit for most drivers is 0.08. For commercial drivers, the limit is 0.04.
This doesn't mean that anyone who ever blows more than a 0.08 will get a DUI with no questions asked, though. For example, if a breath test device is not calibrated properly, the charges could be overturned on the grounds that the reading can't be trusted and the person did not seem otherwise intoxicated.
Still, it's important to know that per se laws exist and how they work. If you're facing charges, you also want to know all of your defense options.A Nevada criminal defense attorney can help you choose the best one.
Source: FindLaw, "Per Se DUI Laws," accessed Oct. 29, 2015