A method for detecting illegally parked vehicles includes obtaining information, in the form of a digital parking map, about available parking areas and areas that are unavailable for parking, where the information about areas unavailable for parking includes coordinates, in particular GPS coordinates, and expected lengths of the areas. The length of an area that is unavailable for parking is detected by a parking space searching vehicle and an illegally parked vehicle is detected if the detected length deviates from an expected length.