It was on I66, heading West from DCA, about 5-10 minutes after I left the airport. It wasn't HOA violations because they were doing it on the exit ramp: the police were either waving people ahead or waving them to the side of the road to search them.
Last time I was in DC I took a 3-hour tour with my wife before our flight left on a Sunday. The DOT did a surprise inspection of our bus at the first stop of the tour. There was a 1hour wait for the actual inspection during which we had to stand out in the heat. Then they said there might be an air leak in one of the tires so they wouldn't allow the bus to run again. Then we had to wait another 1 hour for a new bus to come from the company out in the suburbs. The tour ended up lasting 3.5 hours with 2 hours of it standing in the sun waiting on the DOT. Why can't they inspect the buses out at HQ when there aren't a bunch of tourists on the buses?
Ready for the scary part? I thanked the DOT officers for showing all of the foreign tourists that this was really the "land of the free." Later I was bored and pissed off so I pulled out my zoom lens and started taking pictures of the DOT officers doing the inspections. The local DC police officer told me to put my camera away before he arrested me. In most cases I would have ignored the threat and kept shooting, but my wife was with me, our flight was in about 6 hours, we already checked out of our hotel, and I don't hear nice things about DC jails.
Earlier that day I was in the Holocaust Museum and I was struck by a series of photos of Jewish prisoners being marched further into Germany towards the end of the war. The pictures were clearly taken secretly by civilians who were afraid of being caught photographing the authorities. I had no idea I would be afraid to take pictures of the authorities in America's capital later that same day.