Overloaded road vehicles induce road unsafety, deterioration of infrastructure and unfair competition in freight transport. The European Directive 96/53 (revised 2015/719) regulates commercial vehicle weights and dimensions for international traffic and requires Member States to carry out checks and report on them. Weigh-in-motion (WIM) systems currently detect overloaded vehicles to screen them prior enforcing them on approved static scales. The work carried out since 2015 in France paves the way for direct enforcement of overloads by WIM, which would make it possible to enforce 200 times more of overloaded vehicles, with less staff and resources. This will ensure a better compliance, and ensure a greener and fairer transport market, and increased road asset durability and road safety. The French Ministry of Transport is in charge of the deployment of direct enforcement, in partnership with the Ministry of the Interior for legal issues, and with the Legal Metrology (Ministry of Economy) and its National Metrology and Testing Laboratory (LNE) for certification. The Université Gustave Eiffel and the Cerema carried out research and development works and tests, on open road and on a closed test site, to prove the feasibility of direct enforcement by WIM and prepare a type-approval procedure. The work has focused on assessing the accuracy and reliability of marketed WIM systems. The paper summarizes the main results obtained both with reference and instrumented vehicles and with vehicles from the traffic flow. The aim is to get a right balance between almost no false positive, which would lead to wrong fining, and the lowest tolerance on overloads. The impact of the heavy vehicle dynamics on weighing was assessed, by testing with an instrumented truck, to confirm physical modelling and check that the certification in the required accuracy classes could be achievable.. Extensive tests have been carried out over more than 4 years on the motorway A4 (operated by SANEF) in eastern France. Static weighs were used as references to assess the accuracy of WIM systems. WIM manufacturers providing systems based on piezoquartz sensors by Kistler were pre-qualified, with respect to the accepted tolerances and levels of confidence. A certification procedure including type-approval, initial and in-service verification, has been developed, based on the OIML recommendation R-134, with a few proposed and justified derogations. A test site, Transpolis near Lyon, was pre-qualified for the typeapproval tests. The results show that marketed WIM systems meet the OIML class 10, for fully loaded or overloaded trucks, and the class 5 is achievable for the heaviest trucks. The class 10 is also achievable for the gross weights of the light commercial vehicles (LCVs, below 3.5 t). The certification procedure is now being approved by the LNE.