NYSRPA checked this. NYS police did not have roadblocks set for this event. False rumor.
DC has private liquor stores ~ they have sales ~ they have lower liquor taxes ~ or did for many years ~ Congressmen buy their booze in DC so it'd best be low.
People from Maryland would drive over to DC to buy booze. Maryland cops would hang out to watch what they bought, then, sometimes, they'd follow them home, or, if the guy bought enough ~ they'd know he was buying for a business ~ and they'd just show up at his business and bust him. Ya' gots' to have them thur murland stamps on ye bottles!
Only rarely did they tag someone for his personal purchases ~ just wasn't worth it.
Currently they seem to be out of that business. Both Maryland and Virginia tightened up some of the laws regulating out of state cops when we found out Mayor Bloomburg was using NYC cops to run guns. Currently he can't even fly into Dulles or Reagan airports for fear of arrest. He has to go to Baltimore!