Agent-based traffic simulation has become more and more attractive and important to develop new ITS (Intelligent Transport Systems) services. So far a variety of studies and developments that combine simulators and evaluate ITS services on the combined simulators have been conducted. In this paper, we introduce a simulation environment, called Agent-based USE (Agent-based Unified Simulation Environment), and some application examples for ITS services. The Agent-based USE provides an easy-to-build simulation environment for ITS-related services. In particular, by connecting simulators with ITS services, the Agent-based USE determines behaviors to be changed on the simulators using the data of the services such as recommendation results generated by the services, tells the decisions to simulators; the Agent-based USE then obtains the data representing the current situation on the simulators and sends the data to the services as feedback so as to enable the services to generate the next recommendation. In addition, by using the Agent-based USE, it is possible to construct a co-simulation environment where simulation is performed by synchronizing some kinds of simulators and services and by sharing each simulation information. In this paper, we introduce the overview and architecture of the Agent-based USE for traffic simulation, and discuss its usefulness through some application examples.