Prior to 2004, the standard in Pennsylvania case law to stop a vehicle was probable cause if an officer suspected a violation of the vehicle code. The amendment in 2004 set the standard at reasonable suspicion. This aligned Pennsylvania with the Supreme Court case that outlined a Terry stop.
So, for DUI's, the same rules apply for any stops in Chester County. In these cases, a Chester County DUI lawyer should know what constitutes reasonable suspicion and probable cause and make sure the officer followed the law accordingly.
The Pennsylvania Supreme Court found that this lower standard of reasonable suspicion does not violate either the federal or Pennsylvania constitutions. It noted that the previous hot heightened standard which was probable cause was only statutory. This was enlightened by the court in Commonwealth v. Chase of 2008. This new holding is consistent to what the Superior Court had held at least for DUI cases. The case of particular importance is Commonwealth versus Ulaan. In this case, from 2006, an experienced officer observed a motorist "buzz" through an emergency zone at approximately 60-65 mph where the limit was 35 mph. It was held that the officer had reasonable suspicion to stop the vehicle. In addition, in 2006, in Commonwealth v. Smith, the Superior Court held that reasonable suspicion may also be supplied by anonymous tips.