In addition, people "parked" in a lane of travel might be waiting for a spot to open that someone just got in to their car. The car waiting is essentially double parked though at least still in the car. The appropriate response is the same however - hit the horn in frustration in hopes the driver is within earshot, then try to merge into a lane of traffic that is moving.
And for all those West Coasters to relate - it's like going 50mph down the PCH and coming upon a car fully stopped in the right lane waiting for the guy in his wetsuit on the shoulder to finish changing so he will move his car and open up a parallel parking spot.
