Eric Barker rummaged up an excellent old study about the territorial behavior that drivers show when they see you waiting for their parking spots. The counterintuitive takeaway is that drivers actually take longer to leave a spot when someone is waiting for it than when no one is waiting for it.
The study is from 1997, but let's be honest, who knew there was an empirical answer to the very important question, "Is it me or is this #$*&% taking an extra long time getting out of this spot?"
Three studies showed that drivers leaving a public parking space are territorial even when such behavior is contrary to their goal of leaving. In Study 1 (observations of 200 departing cars), intruded-upon drivers took longer to leave than nonintruded-upon drivers. In Study 2, an experiment involving 240 drivers in which level of intrusion and status of intruder were manipulated, drivers took longer to leave when another car was present and when the intruder honked. Males left significantly sooner when intruded upon by a higher rather than lower status car, whereas females' departure times did not differ as a function of the status of the car.