Three road bridges of prestressed concrete were explored experimentally and theoretically (the slab bridge over Radbuza river at Pilsen town, the prestressed concrete frame box girder bridge over Sedlice’s creek on the highway D1 and the prestressed concrete frame box girder bridge near Koberovice village on the highway D1). This paper contains the results of experimental modal analysis of the observed bridges, which were periodically repeated, comparison of the results of modal analysis results from several measuring periods in terms of the changes of the natural frequencies, the MAC_(j,j) coefficients, the COMAC_(p) coefficients and the changes of the modal flexibility matrix [d], the analysis and estimation of causes of changes.