The Fourth Amendment to the U.S. Constitution protects people against unreasonable searches and seizures by law enforcement, but the right to go about your daily activities without interference from Police is not without its limits. One common situation in which where your rights under the Constitution and the ability of the police to carry out their duty to enforce the laws come into focus is during routine traffic stops.
Common situations in which police stop vehicles

Police must have probable cause to believe a crime has been committed in order to stop you or search your vehicle. Seeing you commit a traffic law violation is a common cause for a traffic stop. Checkpoints are an exception to the rule requiring probable cause to stop a vehicle because they are authorized under Pennsylvania Law.
Grounds for searching a stopped vehicle

Anything a police officer observes from outside your vehicle using only his or her five senses can be seized and could give probable cause for searching the vehicle. For example, a gun left in plain view on the back seat or the smell of marijuana as you roll down your window can be enough to justify a further search. A police dog trained to detect the scent of drugs or explosives could be the basis for probable cause for a search and search if the animal picks up a scent from outside a vehicle.

A mistake made by many people is giving consent to a vehicle search. The Fourth Amendment requirement of probable cause does not apply when you give police permission to search, so anything they seize may be used as evidence to prosecute you for committing a crime.

Consult an attorney

Decisions from the U.S. Supreme Court and Pennsylvania courts are constantly expanding and restricting the rules under which police may conduct searches and seizures of vehicles they stop. You should waste no time contacting an experienced Pennsylvania criminal defense lawyer if you are charged with a crime based upon a search and seizure.