In order to conduct a search of a person or premises, police officers must obtain and present a valid warrant. A judge will issue a warrant based on the information presented in an affidavit from a police officer. The affidavit must show that there is probable cause to believe that criminal activity has taken place and that a search will turn up evidence of the crime. Even if a judge approves a warrant and police conduct a search, you can challenge that the warrant did not establish probable cause, which would allow you to suppress evidence from the search.

Establishing Probable Cause

A police affidavit must describe in detail what they are searching for and why they believe that a crime has been committed. For instance, a police officer can request a warrant to conduct a blood test based on evidence that they reasonably believe a driver is under the influence of alcohol. When requesting to search a private residence, the affidavit must show probable cause by:

• Presenting objective evidence of criminal activity at the residence and involving the accused parties; and
• Establishing the credibility of the source of that information.

A police officer who claims to have witnessed the alleged criminal activity is generally considered a reliable source, based on their experience in such cases. Other sources are less reliable, particularly when they are police informants who may be providing information in exchange for leniency on their own criminal charges.

A cornerstone principle of criminal law in Illinois is that an officer must have probable cause to believe a person has broken the law or is in the process of breaking the law before that person can be placed under arrest. This principle holds true in Illinois driving under the influence cases as well. Before placing a suspected drunk driver in handcuffs, the arresting officer must possess facts that would suggest to a reasonable person that the suspect has in fact been operating a vehicle under the influence of alcohol. Many Illinois officers rely on the presence of one or more indicators or clues to support this probable cause finding.

Indicators of Intoxication While Driving

When asked for the facts or circumstances which justify the arrest of a suspected drunk driver, officers will routinely go back to indicators of impairment that the officer may have seen while the suspect was driving his or her vehicle. These indicators may include:

The Fourth Amendment to the U.S. Constitution guarantees protection from unreasonable searches and seizures by police officers. Generally speaking, in order to conduct a search of a person, his or her car, or his or her residence, the police must either have a warrant authorizing the search, or have probable cause to conduct the search. In fact, warrants should not be granted permitting a search if there is no probable cause. Yet what exactly is probable cause?

Probable cause for an arrest is found when the police have a reasonable basis to believe that a person has committed a crime. In a search, the police have to have a reasonable basis to believe that they will find evidence of a crime from the person or place to be searched.

What constitutes the reasonable basis is often settled by a judge during a preliminary hearing after the search or arrest has already taken place. If a judge decides that there was no reasonable basis for the search, any evidence of a crime that was found during the search cannot be used against the defendant in a criminal prosecution against them. If the police can find independent evidence of the defendant's wrongdoing through other avenues however, the defendant can be prosecuted for the crime.

The police in America often appear to wield an unchallengeable authority. However, the law in fact places numerous constraints on the police to protect the rights of citizens accused of crimes. One of the most important of these constraints comes from the Fourth Amendment, which protects citizens from “unreasonable” searches by police. This means that there are limits to the ability of the police to look through a person's things. The full extent of the rules on when police may search are complex, but a short primer can help people more fully exercise their rights. The general rule is that police must have a warrant to perform a search, but there are important exceptions to this rule.

The Need for Warrants

The key point of the Fourth Amendment is that police must have a warrant to in order to perform a search. A warrant is a legal document provided by a judge that gives the police a right to enter a specific place in order to look for a specific item. In order to get a warrant, the police must demonstrate that there is “probable cause” to believe that a search will turn up evidence of a crime. If the officers can convince a judge of that, then they will receive a warrant, and they can search the premises.

Traffic stops are a common way that ordinary citizens find themselves interacting with law enforcement. In fact, statistics compiled by the researchers at the University of Illinois at Chicago reveal that over two million traffic stops occur every year, just in Illinois. At that rate, every adult in Illinois would average a traffic stop once every four years. With such a high frequency, it is important for drivers to know their rights in order to make sure they can exercise them fully during the stop.

Permissible Stops

The first thing people should understand about traffic stops is that police are only permitted to stop drivers under certain circumstances. Officers may not simply choose to arbitrarily pull cars over. This is because people have a constitutional protection against unreasonable searches and seizures, and stopping a driver on the street legally qualifies as a seizure.