Within the coming years, the developments of infrastructural and in-vehicle ITS systems will come together and result in the development of cooperative vehicle-infrastructure ITS systems. The development of these integrated cooperative systems will require new modelling tools that allow a detailed modelling of both the roadside and the in-vehicle systems and especially the cooperation between them. In this paper, such a modelling tool is presented: the ITS modeller. Within the ITS modeller, intelligent roadside and vehicle systems can be easily implemented and tested. It is equipped with ready-to-use evaluation modules for throughput, safety, noise and emissions.