People often think that someone who is under the legal Blood Alcohol Concentration limit of 0.08 is completely safe to drive, while someone over the limit is a serious danger to society.
The reality is that, while the limit exists for a reason, it's not a firm line that defines safety. Does it really make sense that a person whose BAC is 0.07 is that much safer than someone whose BAC is 0.08? The difference is tiny.
In fact, DUI laws reflect that. Technically, the police can make an arrest if the person is too drunk to drive, regardless of his or her BAC. They'll often use other factors to determine that.
For instance, perhaps an officer watches someone swerve back and forth on the highway, darting over the center line. The car is also driving at night, with the lights off. The officer tries to pull the car over, but he or she can't get there fast enough. As the officer watches, the car swerves into oncoming traffic and causes a head-on collision.
When the officer gets there, the offending driver has red eyes and smells like alcohol. A breath test comes back at 0.07. The officer certainly has enough other information to determine that the person was impaired and can make an arrest.
That's what DUI laws center on: finding those who are impaired by alcohol. BAC is just one way to decide if someone is impaired.
This is important to remember because alcohol use can help determine fault in an accident. Those who are hurt may then be able to seek financial compensation for their injuries.
Source: FindLaw, "DUI Offense Basics," accessed Dec. 29, 2017