Cooperative speed harmonization based on floating car data aiming at improving manoeuvrability in a highly utilized intersection is presented. Cooperative Intelligent Transportation Systems (C-ITS) aims at gather information about the current traffic situation based on wireless communication and provide aggregated information back to the road users in order to improve e.g. efficiency, safety and/or comfort. Simulations show that the proposed speed harmonization application is capable of lowering the CO2 emissions with up to 11%, increasing the average speed with up to 14% and reducing the travel time with up to 16% for all vehicles in the simulation. It is also found that not only the cooperative vehicles benefit from the application but also the non-equipped vehicles. Furthermore, the cooperative traffic simulator has been shown to be a valuable tool for investigating how C-ITS applications may be utilized to develop future traffic system.