When you steal or hijack a police car you can enter the police computer to get
access to a list of current crimes. The criminals will be shown on your
map once you select which crime you want to stop. You can not leave your police vehicle until you arrive at the location and the targets shows on your map.
You will have to complete at least 20 different missions if you want to make 100% of the game. The vigilante situation can be all from chasing a car or taking out a gang of criminals, much like the Most Wanted missions.