The jurisdiction in a DUI case is where the DUI was effectuated; which is essentially where the police pull you over and make an arrest. The local police, such as Baltimore County or Howard County Police are charged with the responsibility to protect their given jurisdiction; they are not authorized in normal DUI/DWI cases to travel into adjacent jurisdictions to arrest except in special circumstances. The Maryland State Police conversely have jurisdiction throughout Maryland and therefore are not limited by County.
I was recently hired to defend a DUI case in Baltimore County. The arresting officer (an off duty Baltimore City Cop) was driving home and stopped my client in Baltimore County. After the alcohol stop he called the local police who came and proceeded with the arrest, field tests, etc. The defendant was taken to the Baltimore County Police station and blew an illegal blood alcohol content whereupon he was charged with DUI, DUI and related offenses.
At the trial of this Defendant I explained to the State's Attorney that they had a jurisdiction issue. That an off duty Baltimore City police officer may not effectuate a stop in Baltimore County because he lacks the authority, he must instead call the local authorities or a State Trooper to make the stop. That is to say, an off duty cop out of his jurisdiction has the same rights that a regular citizen has. If he exceeds those rights, as he did in this case, the stop and subsequent arrest become illegal. Once you have an illegal stop, any evidence flowing from that illegal stop may be suppressed. Once the state or the Judge agrees, the case can be dismissed, as it was in this instance.
When defending a DUI charge, there are many nuances both prior to the actual arrest and the procedure of the arrest itself that must be properly identified by experienced DUI counsel in order achieve the best possible result in any given case.