Police can’t search your property for evidence or put you in custody without a search or arrest warrant respectively. At least, that’s usually the case, but the law can arrest someone without the necessary warrant—called a warrantless arrest—as long as it’s considered “reasonable” by law. This is where the Fourth Amendment comes in. While it prohibits searches and seizures without warrants, there are a number of exceptions to the rule.
First, an officer doesn’t need a warrant of arrest if the crime—felony or misdemeanor—was committed right in front of him. This generally applies to DUI, grand theft auto, and other crimes so long as there’s probable cause. Furthermore, an officer doesn’t need a search warrant if the criminal had been arrested. For example, after a highway chase ends with the offender being apprehended, police can search his car for evidence that can be used against him.
In fact, most of the conditions for a warrantless arrest or search involve the road and public areas. Notice that in high-speed police chases, police are able to arrest their man without an arrest warrant and search his car even without a search warrant. Unless the police can prove the conditions necessary for a warrantless arrest or search, they shouldn’t be able to touch you otherwise.