## 2 Background
### LPR Systems  
LPR systems are automatic image processing systems that recognize the license plate number of a vehicle. Such systems work on one or more camera-taken pictures that may be of the color, black-and-white, or infrared type [7]. LPR systems combine several techniques to obtain the license identifier, such as object detection, image processing, and pattern recognition. After the license plate number is obtained, it can be associated with data stored in databases, and this crossing of information enables other analyses for a diversity of applications. Examples include electronic payment systems (toll payment, parking fee payment), monitoring systems that identify road traffic intensity, and surveillance and monitoring of individuals and vehicles [4; 7].  
When an LPR system detects a vehicle and recognizes its license plate number, this number is compared to vehicle database records of interest in criminal investigations. In the case of a suspect vehicle is identified, a law enforcement officer can intercept and stop the vehicle, check for evidence, and, whether necessary, make arrests. However, all the license plate numbers of vehicles passing through a camera are stored, even the ones of non-suspect vehicles [8]. As the LPR system of a city can recognize and register hundreds of license plate numbers every minute, it stores large amounts of data. These data are stored for a period for use in future investigations (the legislation of each country determines this period. Thus, when a vehicle of interest is registered in the system, the authority can perform retrospective analysis and identify possible locations of an investigated suspect based on the movement history of his/her vehicle.  
The storage of data regarding vehicles that are not of investigative interest has raised concerns about the privacy of citizens. On the one hand, police forces around the world claim that the use of LPR systems has increased the power of crime prevention and aided investigations. On the other hand, civic organizations and ordinary citizens have questioned whether LPR systems protect the personal data associated with the identified license plates. They worry that such information could be used for purposes unrelated to public safety. In any case, the use of LPR systems by police forces has increased significantly around the world, under different justifications [9].  
According to the authors of [4], the first adherents of LPR systems were the police departments and government agencies of the United Kingdom (UK), around 2002. Then, the UK Home Office, a ministerial department of Her Majesty's Government of the United Kingdom responsible for immigration, security, and law and order, began to build and evaluate strategies for using LPR systems. Research indicates that, by 2006, all police forces in England and Wales had LPR technology. Over the same period, many similar technologies related to vehicle traffic monitoring were already available, including radar and traffic light cameras, as well as toll cameras. UK law enforcement agencies use LPR systems within a nationally interconnected infrastructure that centralizes the storage of captured data in a single data center, which is governed by standards for the use of technology [10].  
In the United States, LPR systems were introduced around 2004 and quickly gained popularity in law enforcement circles. In 2007, the International Association of Chiefs of Police (IACP) established a resolution promoting the use and purchase of an LPR system with federal funds [11]. At that time, a survey conducted by the US Department of Justice estimated that about 19% of agencies with more than 100 employees were using LPR [12] systems. A survey published is 2013 shows that this percentage increased to 66% in that year [13]. A new research was conducted in 2016, but until this moment (2020), the results of this research had not yet been disclosed.