The article describes the requirements for automatic weighing systems for vehicles in motion. These systems can be divided into two types – preselective WIM and accurate weighing systems HS-WIM, which do not require vehicles to be stopped. The metrological requirements, the scope of data collected in a weigh-in-motion system, and the legal solutions that form the basis for imposing a penalty are key to the effective imposition of penalties. Accordingly, the article describes the requirements for WIM systems used in procurements carried out by the General Directorate of National Roads and Highways, as well as those specified in the COST 323 standard, set forth by the International Society for Weighing Vehicles in Motion – ISWIM (based on assumptions developed by the International Organization of Legal Metrology, contained in the OIML R 134-1 recommendation), and those set forth in the American ASTM E1318-09 standard. The authors describe an example of the HS-WIM system operating in Hungary, giving the legal basis, the way the system works and the scope of data collected and processed. The article formulates the thesis that the correct operation of the HS-WIM system is primarily due to appropriately designed legal provisions, which are the basis for imposing penalties and specifying the principles of legalization of weighing systems, and the method of managing the system and flexibility in its implementation can have a fundamental impact on whether the system will be permanently introduced in a given country.