The complexity of cooperative systems makes the use of high fidelity simulation essential in the development and testing of cooperative applications and their interactions with other cooperative systems. In SAFESPOT a simulator test bench is setup to test the safety margin applications running on multiple vehicles and road side units. This paper presents the MARS simulator, its integration in the simulator test bench, and how it is used to test the safety margin concept in safety critical situations. MARS is a high-fidelity multi-agent simulator that models the behavior of individual hardware and software components. MARS enables the simulation of a virtual environment, including its interaction with other application units, sensors, data fusion, communication and the Local Dynamic Map, for testing a single application unit. The extensions to MARS provide the flexibility to scale up the virtual environment to partially and fully integrated applications. This results in a test bench that supports collaborative development and testing of cooperative systems up to the system integration phase. ©2010 IEEE.