Police Make an Unconstitutional Traffic Stop: What Happens If They Find Evidence of DUI?

In 2017, Pennsylvania State Police made 19,963 DUI arrests. That number does not include DUI arrests made by local police departments.

With a staggering number of people arrested for DUI, there are bound to be some traffic stops that are made without probable cause, in contravention of a driver’s constitutional rights. So, what happens to the DUI evidence when police make an unlawful traffic stop? The Superior Court of Pennsylvania recently answered that question.

Failure to Use a Turn Signal

In Commonwealth v. Staples, a police officer stopped the driver of a BMW when he made a right turn from a parking lot onto a public roadway without using his turn signal. During the stop, the officer observed that the driver exhibited signs of intoxication including inexplicable laughter, difficulty maintaining his composure, difficulty following instructions, inability to answer simple questions, and slurred speech. The officer also detected a strong odor of alcohol. The officer administered a field sobriety test, which the driver failed.

The driver was charged with DUI-general impairment and DUI-highest rate of alcohol. He sought to suppress the evidence, arguing that the officer lacked probable cause to stop his vehicle.

Pennsylvania Vehicle Code

In Pennsylvania, police have probable cause to stop a motor vehicle when they observe a traffic code violation, even if it is a minor offense. Here, the trial court found that the driver’s failure to use a turn signal did not violate the state’s Vehicle Code. Specifically, the code requires use of a turn signal on a “roadway.” Under the code, the parking lot from which the driver began his turn is considered to be a “trafficway,” and thus, the court reasoned, he was not required to use his turn signal. Accordingly, the court suppressed the DUI evidence gained from the traffic stop, finding that the stop violated the driver’s constitutional rights.

The Commonwealth appealed.

Plain Language

On appeal, the Superior Court relied upon the plain language of the Vehicle Code to affirm the trial court’s ruling:

[The statute] provides that “[u]pon a roadway no person shall turn a vehicle … unless and until the movement can be made with reasonable safety nor without giving an appropriate signal in the manner provided in this section.” … Under the plain meaning of this statute, the requirement of the use of a turn signal applies only where a car is “upon a roadway” when starting the turn. Here, because [the driver] was on a trafficway when he started his turn, he did not violate [the statute]. Accordingly, the trial court did not err in finding the police officer lacked probable cause.


Without the ability to use the DUI evidence obtained during the traffic stop, the Commonwealth most likely will not be able to proceed with its case. This case serves as an important reminder: DUI cases can rise and fall based upon police conduct alone, without getting into issues of driver impairment. As always, if you have been arrested for DUI, we recommend that you consult with a qualified DUI attorney immediately to ensure that your legal rights are protected.