Embedded software for modern automotive and avionic systems is increasingly complex. In early design phases, even when there is still uncertainty about the feasibility of the requirements, valuable information can be gained from models that describe the expected usage and the desired system reaction. The generation of test cases from these models indicates the feasibility of the intended solution and helps to identify scenarios for which the realization is hardly feasible or the intended system behavior is not properly defined. In this paper we present the formalization of requirements by models to simulate the expected field usage of a system. These so called usage models can be enriched by information about the desired system reaction. Thus, they are the basis for all subsequent testing activities: First, they can be used to verify the first implementation models and design decisions w.r.t. the fulfillment of requirements and second, test cases can be derived in a random or statistic manner. The gen- eration can be controlled with operational profiles that describe different classes of field usage. We have applied our approach at a large German car manufacturer in the early development phase of active safety functionalities. Test cases were generated from the usage models to assess the implementation models in MATLAB/Simulink. The parametrization of the systems could be optimized and a faulty transition in the implementation models was revealed. These design and implementation faults had not been discovered with the established test method.