The various names for "driving under the influence" (DUI)—"driving while intoxicated" (DWI), "operating while impaired," and so on—all describe the driver in a state of being too intoxicated to safely operate a vehicle. However, proof of actual driver impairment isn't necessarily required for a DUI conviction.
Each state has laws that prohibit drivers from operating a vehicle with a blood alcohol level that's above a certain limit. (Some states also have similar laws that apply to drugs in a driver's system.) These "per se" DUI laws allow a driver to be convicted of DUI based solely on chemical test results—the prosecution doesn't need to prove actual impairment.
All states have per se DUI laws. But these laws vary across the country in some respects. Generally, per se DUI laws fall into one of two categories:
These are the basics. But, again, the details of state per se laws differ somewhat.
Under per se laws, a driver who has a BAC or drug concentration above the legal limit can be convicted of a DUI without proof of actual impairment. So, for prosecutors, it's typically easier to prove a per se DUI than a DUI charge based on driver impairment. Proving a per se DUI is generally clear-cut and based on chemical test results.
Proof of impairment, on the other hand, normally involves the testimony of multiple witnesses, and in the end, impairment is a subjective determination. So, even with witness testimony, a conviction isn't guaranteed.
To provide the district attorney with evidence to prosecute per se DUIs, police usually make every effort to obtain a breath, blood, or urine sample that shows the amount of alcohol and drugs in the driver's system. In fact, implied consent laws generally require drivers who are lawfully arrested for a DUI to submit to chemical testing.
Although prosecutors generally have an easier time with per se charges than with proving an impairment DUI, drivers still sometimes have viable defenses.
For example, if an officer didn't have a valid reason to stop or arrest a DUI suspect, the evidence obtained following the stop (including chemical test results) generally isn't admissible in court. Without chemical test results, prosecutors don't have what they need to prove a per se DUI charge.
In some circumstances, a driver might also be able to successfully argue that law enforcement violated his or her rights in some other way or that chemical test results were inaccurate.
However, per se DUI defenses are specific to the facts of the case. If you've been arrested for driving under the influence, it's best to get in contact with an attorney who can analyze your case for possible defenses.