The precise amount of evidence that constitutes probable cause depends on the circumstances in the case. To illustrate, assume that a police officer has stopped a motor vehicle driver for a traffic violation. In the absence of any other facts indicating criminal activity by the driver, it would be a violation of the Fourth Amendment if the officer conducted a full-blown search of the driver and the vehicle. The mere commission of a traffic violation is not, in and of itself, a fact that supports probable cause to believe that the driver has committed a crime. However, if the officer notices that the driver's eyes are bloodshot or that the driver smells of alcohol, the officer may detain and question the defendant, search him, and place him under arrest. Most courts hold that a driver's commission of a traffic violation combined with the appearance that the driver has used drugs or alcohol constitute sufficient evidence to lead a reasonable person to believe that the person is driving under the influence of drugs or alcohol.
Probable cause is not equal to absolute certainty. That is, a police officer does not have to be absolutely certain that criminal activity is taking place to perform a search or make an arrest. Probable cause can exist even when there is some doubt as to the person's guilt. Courts take care to review the actions of police in the context of everyday life,
Balancing the interests of law enforcement against the interests of personal liberty in determining whether probable cause existed for a search or arrest.
Legislatures may maintain statutes relating to probable cause. Many such statutes declare that a certain thing constitutes probable cause to believe that a person has committed a particular offense. For example, under federal law, a
Forfeiture judgment of a foreign court automatically constitutes probable cause to believe that the forfeited property also is subject to forfeiture under the federal
Racketeering law (18 U.S.C.A. § 981 (i)(3) [1986]).