If you are charged with DUI - less safe, the state must prove beyond a reasonable doubt that you were driving while under the influence of alcohol to the extent you were incapable of driving safely.
On the other hand, if you are charged with DUI - per se, the state has to prove beyond a reasonable doubt the you drove while your alcohol concentration was 0.08 grams or more (as measured within 3 hours after driving)
And the state can charge you in the alternative, able to get a conviction if they can prove it either way. Penalties are exactly the same.
Bob is absolutely correct. What a lot of people don't realize is that you can be charged with DUI less safe even when you submitted to the state test and your blood alcohol concentration was less than .08.
Prosecutors have been charging both Less Safe and Per Se DUIs for years, ever since the legal limit for intoxication started dropping (from 0.10 to 0.08) years ago. In fact, many prosecutors would prefer that you plead to the Less Safe DUI in case the law changes again sometime in the future or there is some constitutional challenge to the testing device or the per se analysis. As my colleagues have noted already, you can be convicted of DUI Less Safe even when you have less than 0.08 BAC in your system -- all they have to prove is some level of intoxication by drugs or alcohol AND a less safe driving behavior, such as failing to maintain a lane or failing to come to a complete stop at a stop sign. But to answer your direct question, having a 0.08 BAC anytime within 3 hours of driving a motor vehilce from alcohol consumed prior to driving will get you a DUI (over the limit = "Per Se").
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