The Evidence that a Police Officer Needs Before He Can Arrest You
An officer needs Probable Cause before he can arrest you. In the case of DUI, the officer needs Probable Cause that you are or have been operating a motor vehicle while under the influence of alcohol or drugs. Remember that driving under the influence of medication is also a crime. If your ability to operate a motor vehicle is impaired by medication, you can be convicted of DUI. It does not make a difference legally whether the medication is prescribed and whether you're taking the medication as prescribed.
What is Probable Cause?
A police officer needs enough evidence to show that there's a probability that your operation of a motor vehicle is impaired by drugs or alcohol. Probable Cause is a low standard. It's much lower than proof beyond a reasonable doubt. It's even lower than "more likely than not." In other words, a police officer doesn't even have to believe that there's a 51% chance you're operating a vehicle under the influence. The officer just has to show there's some probability.
How Does a Police Officer Get to Probable Cause?
The easiest way for an officer to get to probable cause is with a preliminary breath test. If you agree to blow into a preliminary breath testing device and the result of the test is anywhere near the legal limit (0.08), the officer has Probable Cause to arrest you. That's one important reason you shouldn't blow into a preliminary breath testing device. I have represented people who have not had any alcohol to drink, but still register a positive result on the preliminary breath test because they use mouth wash or breath freshener.
Another way for an officer to get to Probable Cause is Field Sobriety Testing. If you do not perform the field sobriety tests to the officer's satisfaction, he will often have enough Probable Cause to arrest you. Field sobriety tests aren't as good evidence as preliminary breath tests because many people can't perform the tests sober, but they can easily provide enough evidence to arrest you.
Another way for an officer to get to Probable Cause is by your own statements. If you're sitting behind the wheel of a car and you tell an officer that you've had a couple beers within the last few hours, that can go a long way toward Probable Cause. Other things that an officer looks for when developing Probable Cause is slurred speech, bloodshot or glassy eyes, disorientation, inability to follow directions, etc.
As you can see, the most important evidence that a police officer uses to get to Probable Cause is evidence that you voluntarily provide him: preliminary breath tests and field sobriety tests. If you decline to provide a preliminary breath test and to participate in field sobriety testing, the officer will have a much more difficult time getting to Probable Cause. If the officer can't get to Probable Cause, he can't arrest you. It's important to point out that an officer can get to Probable Cause without a preliminary breath test and field sobriety tests, but it's much more difficult.