There are several sets of circumstances that could lead to a motorist being arrested for driving under the influence. One way an arrest may occur is if a police officer has what is called "probable cause." An officer may have probable cause based on the belief that a crime has been, or is about to be committed. In the case of DUI, probable cause may be applied as the basis of an arrest if a suspect either can't or won't submit to a chemical test.
An example of probable cause could be if an officer pulls over a suspect for speeding and then detects the odor of alcohol coming from the car and sees a mostly empty whiskey bottle on a car seat. If the suspect chooses not to take the chemical test, the officer may still be able to carry out an arrest based on the other evidence that the suspect had been drinking and driving.
Another justification for making an arrest could be if an officer actually witnesses a suspect in the act of driving while intoxicated. For instance, if the officer sees a suspect driving erratically and then pulls him or her over to administer a Breathalyzer test. If the test indicates the driver is legally intoxicated, then an arrest may be made.
The above are just a couple of the circumstances under which a driver may be arrested for driving under the influence. However, it is important to remember that the police should never violate your constitutional rights when making an arrest. So if you are arrested for DUI, you may want to contact a defense attorney who can go over the record of your case. If the attorney can find evidence that the police did not follow proper procedures when making the arrest, you may be able to have the charges dropped.