That's merely a deterrent. Some police departments (especially the smaller ones) often use this trick along major routes so that people will slow down or just be more motivated to drive safely. Granted, such tricks will only last so long so a car like this will usually not remain in the same spot for too long.

Some agencies also just simply park some of their older cars along the side of the road, without a dummy figure. I saw this when I went to Williams, Arizona back in 2003 (Williams is the tourist town for the Grand Canyon). The local police had a 1987-1990 Caprice parked out on the road leading into town. People sure did slow down for it.