The answer is that it depends upon the violation for which the police are conducting a traffic stop. Pennsylvania law actually states that a police officer can conduct a traffic stop if the officer has reasonable suspicion to believe that a violation of the Vehicle Code has occurred (See 75 Pa.C.S. § 6308(b)). Reasonable suspicion is a slightly lower standard when compared to probable cause, which means that alower burden of cause is required to conduct a traffic stop under reasonable suspicion when compared to probable cause.
While the law says that only reasonable suspicion is required to conduct a traffic stop, the Pennsylvania Supreme Court in Commonwealth v. Chase stated that a stop based upon reasonable suspicion is only permitted if there is an investigative goal for the stop, meaning that the officer has an expectation that the stop will provide additional information concerning the suspected criminal activity. Basically, the stop is justified under reasonable suspicion if there is something to investigate. If there is nothing to investigate, then the officer must have probable cause to justify a traffic stop.
Investigable Versus Non-investigable Offense
Whether or not a police officer needs probable cause or reasonable suspicion to conduct the traffic stop is dependent upon whether the offense is an "investigable offense" or not. For example, if the officer said that the traffic stop was conducted because the driver ran a red light, then the officer must have had probable cause. A red light violation is not an offense that can be investigated, meaning the traffic stop will not allow the officer to investigate whether or not the violation occurred.
The classic example of an investigable offense is a DUI. In most cases like this, the officer conducts the traffic stop in the wee hours of the morning after observing swerving or other erratic driving. The officer conducts the traffic stop to determine whether the erratic driving was a result of the driver texting, changing CDs, or because the person was driving under the influence of drugs or alcohol. During the traffic stop, if the officer smells the odor of alcohol on the person's breath, then the driver is normally given standardized field sobriety tests and possibly a portable or preliminary breath test (PBT). If the driver fails those tests, the driver is often arrested and required to submit to a blood alcohol test to determine if DUI charges should be filed.
Why Does It Matter?
If the officer did not have the required level of suspicion to initiate the traffic stop, then the traffic stop is illegal in that it violates a person's constitutional right against search and seizures. Evidence obtained from an illegal traffic stop is suppressed by a judge during the pretrial phase of a case after the defense attorney files a motion to suppress evidence. Any person charged with driving under the influence should contact an experienced DUI lawyer for a consultation and case review to see if challenges to the case can be raised.