For an Illinois DUI (Driving Under the Influence) arrest to be legal, the police must first have a valid reason to come into contact with you. Under the “reasonable suspicion” standard of Terry v. Ohio, 392 US 1, 20 L.Ed. 2d 889 (1968), a police officer may stop a person in a public place for a reasonable period of time if the officer reasonably infers that the person committed, is committing or is about to commit, an offense, including traffic offenses. (Codified in Illinois at 725 ILCS 5/107-14)
There are exceptions to the reasonable suspicion requirement. For one, if the member of the public engages in a voluntary encounter with the officer, the person has given up his Fourth Amendment privacy rights. Another exception to the reasonable suspicion requirements is the “community caretaking” function. This could include assisting a person whose vehicle is broken down or who appears to be in physical distress. People v. McDonough, 239 Ill. 2d 260 (2010).
Random license plate checks are another exception to the reasonable suspicion requirement. One other example of a valid stop without reasonable suspicion involves roadblocks, provided that certain criteria are met.
However, the police may not act based upon a “mere hunch” of criminal activity. People v. Drewes, 278 Ill. App. 3d 768, 215 Ill. Dec. 445 (3d. Dist. 1996). Therefore, a stop based upon a ‘suspicious vehicle”, or a vehicle leaving a closed business parking lot, or a car parking in a no-parking zone, are illegal stops.
In the case of an illegal stop, the defendant should file a motion to suppress (throw out) the evidence gathered from the moment of the illegal stop. While this does not specifically require the judge to “throw out” the case, it in effect forces the prosecutor to dismiss the charges, as the evidence to prove them has been blocked from being brought forward.
A statutory summary suspension (SSS) becomes a factor in most Illinois DUI arrests. The driver is pulled over, the officer suspects he has had too much to drink and he requests a breath test. If the driver agrees and registers above .08, a suspension of his driver’s license will begin automatically on the 46th day following the arrest (625 ILCS 5/11-501.1(h)) unless the suspension is rescinded (thrown out). And if he does not agree to a test, he will be suspended, for a longer period of time than if he had agreed to a test.
The law sets forth specific grounds (reasons) for which a suspension can be rescinded. Those grounds are at 625 ILCS 5/2-118.1 and are as follows:
You were not properly placed under arrest for a DUI offense as evidenced by the issuance of a Uniform Traffic Ticket;
The arresting officer did not have reasonable grounds to believe you were driving or in actual physical control of a motor vehicle while under the influence;
The arrest officer did not properly warn you of the consequences of taking or refusing to take a breath or blood test;
You did not refuse to submit to the test;
You took the requested test and had a blood alcohol content of under .08.
Illinois DUI Lawyer Blawg

