The police officer's job—besides protecting and serving the public—is to make arrests. In fact, most police officers are tasked with "arrest goals" or "arrest quotas" that they are expected to achieve each week. When a police officer stops you on the street, or in your car, or even knocks on your door at home, chances are good that the officer believes that you have information that will assist in making an arrest. Knowing this, should you speak with the police officer? Should you let the officer search your home or car? And what happens if you don't?
A police officer who believes you have committed a traffic offense can arrest you and, under some circumstances, frisk your passengers for suspected weapons. For example, if you've committed a routine traffic offense such as running a red light, a police officer can order the occupants out of the car, and if the officer has a "reasonable belief" that someone in the car is carrying a weapon, the officer can conduct a short "frisk" or "pat down" of the suspect.
Searching the car. During a routine traffic stop, the officer cannot search your car unless the officer has a warrant, or a "reasonable belief" that there are weapons or other evidence of a crime within the car and that the occupants might destroy the contraband. An officer who doesn't have probable cause for searching cannot use the traffic stop as a pretext for an extensive search. If the officer has probable cause, the officer can search the car and the occupants' possessions without a search warrant.
Emergencies. During certain emergencies—for example, a car accident with injured motorists—a police officer may justify a warrantless search of a car. An officer at a routine traffic stop can also briefly question the driver and passengers about matters unrelated to the traffic stop.
Checkpoints. Police officers can stop and question all drivers on a particular traffic route for sobriety, narcotics, or for illegal immigrants (near border crossings). These so-called "checkpoints" are legal, provided that the police follow the same procedures with respect to all motorists on a route—that is, they don't discriminate by stopping, say, only those drivers who appear to be of a particular ethnicity.
Read more about speaking to police after you have been pulled over.
If a police officer comes to your residence and wants to question you, you don't have to let the officer in or answer any questions. You don't have to consent to any searches unless the officer has a search warrant or can justify the search on an emergency basis, as explained below.
Plain view. Of course, you are always free to speak with a police officer and there are many reasons why it may be in your best interest to do so—for example, you were the witness to, or victim of a crime. However, keep in mind that if a police officer enters your residence with your consent and sees any contraband or evidence of criminal activity that is in plain view, the officer can seize that material and use it in a criminal proceeding, even though he didn't have a search warrant. By the way, the police don't need a warrant to search any trash that you leave out for collection.
Emergencies. Officers can enter residences without a warrant in certain emergencies, for example, if the officer is in "hot pursuit" of a suspect who runs into the residence, or if a police hears shouts and screams for help, or if the police officer is responding to a complaint and sees evidence of criminal activity. This right is granted because the police officer has an obligation to protect the public and to preserve evidence.