All states have two types of DUI charge. A person can be convicted of a "per se" DUI based on driving with a certain amount of drugs or alcohol in his or her system. And a person can be convicted of a DUI for driving while actually impaired by drugs or alcohol.
To get a per se DUI conviction, the prosecution generally just needs to show the driver had a blood alcohol concentration (BAC) of .08% or more while at the wheel. Some states also have per se drug DUI laws that apply the same concept to controlled substances. For instance, a state might make it a per se drug DUI to drive with .02 milligrams or more of cocaine per liter of blood in your system.
An impairment DUI charge is based on how the driver was affected by the drugs or alcohol ingested. States have different standards for what counts as "under the influence" or "intoxicated." Some states define an impairment DUI as driving while "substantially impaired" by drugs or alcohol, while other states only require proof that the substances ingested had "some effect" on the driver.
Police typically use three methods for determining whether a driver is under the influence of drugs or alcohol: general observation, field sobriety tests (FSTs), and breathalyzers.
Observations. Generally, an officer's decision to conduct a DUI stop or investigation starts with an observation indicating possible driver impairment. An officer might see the motorist swerving or driving unusually slow. Or while conducting a traffic stop, an officer might note the driver has bloodshot eyes, smells of alcohol, or is behaving strangely. An innocuous explanation might exist for an officer’s observations. But, typically, an officer who notices anything that could signal intoxication will at least want to investigate to be sure the driver doesn't pose a danger.
Field sobriety tests. While conducting a DUI stop, an officer might ask the driver to perform field sobriety tests (FSTs). Officers use lots of different FSTs. But the most common ones are known as the "standardized" FSTs: the horizontal gaze nystagmus, walk-and-turn, and one-leg stand. Police rely on FSTs to gauge the level of a person's intoxication. Poor FST performance can persuade an officer to make a DUI/DWI arrest. Generally, a driver isn't required to participate in FSTs and can't be formally penalized for refusing.
Chemical testing. At roadside, an officer might ask a driver to take a breathalyzers test. Typically, a pre-arrest breath test is optional—meaning the driver is free to refuse. The breath-test machines officer use in the field are sometimes called "preliminary alcohol screening" (PAS) devices. PAS results give police an idea of what a driver's blood-alcohol level but are generally less accurate than breath-test machines—called "evidential breath test" (EBT) devides—that police use post-arrest, once the driver has been taken to the police station.
Every state has "implied consent" laws that require all motorists lawfully arrested for driving under the influence to submit to an officer's request for DUI chemical testing. Typically, the test will be of the breath or blood, but occasionally law enforcement also uses urine testing. And the laws of most states allowed the officer to select which test the driver must take.
Refusing an officer lawful request for DUI chemical testing typically leads to license suspension. And if your case goes to trial, the prosecution usually can tell the jury that you refused to take a DUI test.
The laws of some states allow motorists to consult with an attorney prior to deciding whether to submit to DUI testing. But these states usually give drivers a limited amount of time to do so. For instance, some states give drivers 30 minutes from the time of arrest to contact an attorney. After the 30 minutes has passed, the officer is going forward with testing whether or not the driver has been able to talk to a lawyer.
The laws of most states, however, don't give motorists the chance to speak with an attorney before deciding whether to comply with DUI testing.
Police must give Miranda warnings before questioning a suspect only if the suspect is in "custody." So, if police have you under arrest with handcuffs on your wrists in the back of a police car, a Miranda warning is probably required prior any incriminating questions. But on the other hand, it's usually okay for an officer who pulls you over for a traffic violation to ask questions—even possibly incriminating ones—without a Miranda advisement so long as the officer hasn't restricted your freedom in any substantial way (like by handcuffing you).
Though DUI charges seem straightforward, that's not always the case. DUI laws are complex and constantly changing. And the facts of every case are different. So, you're almost always better off getting a DUI lawyer to review your case and represent you in court. A qualified lawyer will be able to look at the circumstances of your case and identify the possible defenses.
If you can afford a private attorney, you'll likely be well served to find someone who specializes in DUI cases. Lots of attorneys offer prospective clients a free consultation where you can get some of your questions answered. But even if you don't have the resources to retain a lawyer of your choice, public defenders typically have quite a bit of DUI experience. Whatever your situation, it's generally not a good idea to represent yourself.
(Read more about things to consider before deciding whether to hire a DUI attorney.)
Police typically need probable cause to believe you've broken the law before they're allowed to stop your vehicle. But sobriety checkpoints are an exception to the general rule. In other words, police are generally allowed to set up DUI roadblocks where they stop all motorists that approach.