Time-resolved measurements of the flux-flow voltage provide detailed information on the spatial current distribution and the migration of flux domains across superconducting thin-film samples. Computer simulations based on the Gibbs free-energy-barrier model achieve excellent agreement with the experimental voltage signal generated by a single flux domain. In order to describe the more general case of a series of consecutive flux tubes, the mutual interaction between the flux tubes has to be included. For type-I superconductors the comparison of such extended model calculations with the experimental results reveals a damped oscillation of the number of flux quanta in consecutive domains. Thus, a detailed understanding of the onset of flux-flow is achieved for the first time.