What evidence is needed to arrest someone suspected of drunk driving?

Despite all of the TV shows in which the police always say "you are under arrest," for purposes of a defendant's rights under U.S. laws, an arrest may occur long before those words are uttered. The courts have concluded that an arrest occurs when a reasonable person would conclude that his or her freedom has been significantly limited and that the suspect reasonably believes he is not free to leave.

Generally speaking, there are four types of evidence that a police officer will consider and gather in a drunk driving investigation: gross observations of behavior in general; specific observations of what are known as "field sobriety tests" (FST); information obtained from questioning the suspect; and chemical test results of the motorist's blood, breath or urine.

A police officer may arrest a motorist for drunk driving if the cumulative effect of the evidence convinces the officer that he has "probable cause" or "reasonable cause" to make an arrest. This is a far lower standard than the one the state must prove at trial where the case must be proven "beyond a reasonable doubt." Although this is a high standard, it is met every day in courts all over the country.