Another category of exception to the warrant requirement is the automobile exception. Generally, a police officer can search any part of the car that they have probable cause will uncover evidence of a crime.
It dates back to the 1925 case Carroll v. United States. Despite its rather long history, it is is one of the more frustrating exceptions. The Court originally created the exception because of the mobility of cars and how easy it would be to make evidence disappear if officers waited for search warrants. The exception has expanded so now officers do not have to show there was a risk of evidence being destroyed, and an officer can search areas a weapon could conceivable be reached.
Back in 1925 when the Court crafted the exception, automobiles where a luxury that only a few people owned. Now, they are so ubiquitous that most adults own one. Many people treat their cars similarly to how they treat their homes. They keep clothes, food, toiletries, and countess other personal belongings inside. The Court, however, still treats the automobile as a unique technology that requires a special area of the law. Indeed, even mobile homes someone currently lives in are subject to the automobile exception.
Because a judge may second guess whether the officer had probable cause at the time the search, it is standard practice for officers to ask for consent to search a car; a warrant is never needed if a person voluntarily consents to a search. Even without consent, an officer with probable cause can use the automobile exception.
One may ask themselves at this point, “What difference does it make if I refuse consent, if the police officer can just search my car anyway? First, assuming the person is innocent of any wrongdoing, then the encounter ends sooner. For people who are completely innocent, it is more likely that the police lack probable cause to search the car and will have to let them go. If someone tells the police to go ahead and search, the search can last for upwards of an hour. Second, for people who do have something to hide, by refusing consent they have given their lawyer something left to argue: namely the officer lacked probable cause to search the car.
Consider the most common form of police encounter: the speeding ticket. When an officer stops someone for a speeding ticket, the chances of there being probable cause to search the car are very low. An officer can search a person to secure their safety, but they will need some evidence to show that they had reason to fear for their safety. Furthermore, the officer is not going to be able to search the trunk, a locked glove box, or other areas where neither the driver nor passengers can reach because there will be no evidence to uncover. All the evidence of speeding has already happened. If the officer searches anyway, there is a good chance a judge would suppress the evidence.
There have been some hopeful steps recently. A 2009 case, Arizona v. Gant, was the first case in a long time that narrowed the exception. The defendant in that case had already left his car when the police approached him and arrested him. The Court ruled it was illegal to search his car under those circumstances. It was a small step, but generally the Court had a history of making the automobile exception larger and larger.