The purpose of this study is to present the computational method applicable for stability analyses of structures. In this paper, a typical complex eigenproblem for a monodromy matrix is solved in order to perform the stability analysis of vehicle streams moving along the bridge. The inertia forces of vehicles and the springing of vehicles are taken into account. Unstable structure vibrations were detected. The instability is a combination parametric resonance one. The combination parametric resonance is not a typical one as it involves the interaction of not two or three but numerous modal forms simultaneously. (C) The Authors. Published by Elsevier Ltd.