Cooperative ITS systems are expected to improve road traffic safety and efficiency, and provide infotainment services on the move, through the dynamic exchange of messages between vehicles, and between vehicles and infrastructure nodes. The complexity of cooperative ITS systems and the interrelation between its components requires their extensive testing before deployment. The lack of simulation platforms capable to test, with high modelling accuracy, cooperative ITS systems and applications in large scale scenarios triggered the implementation of the EU-funded iTETRIS simulation platform. iTETRIS is a unique open source simulation platform characterized by a modular architecture that allows integrating two widely adopted traffic and wireless simulators, while supporting the implementation of cooperative ITS applications in a language-agnostic fashion. This paper presents in detail the iTETRIS simulation platform, and describes its architecture, standard compliant implementation, operation and new functionalities. Finally, the paper demonstrates iTETRIS large scale cooperative ITS evaluation capabilities through the implementation and evaluation of cooperative traffic congestion detection and bus lane management applications. The detailed description and implemented examples provide valuable information on how to use and exploit iTETRIS simulation potential.