Since the 1970s, many researchers have attempted to use changes in natural frequencies as means for condition assessment of large civil engineering structures such as bridges, but have faced the challenge of decoupling frequency variations apparently caused by changing operational conditions. In the case of the Tamar Bridge in southwest England, the time series of natural frequencies exhibit diurnal variations resulting from a combination of thermal and vehicular loading, the effects of which would need to be compensated for in dynamics-based assessment. By examining monitored data for several years, the effects of traffic mass have been characterised and compared with other operational effects. While temperature changes appear to have a greater influence for lateral modes, traffic mass is a strong factor in all modes and the dominant factor for the vertical and torsional modes evaluated. Physics-based explanations for the variable effects of vehicle mass have been sought using a finite element model calibrated against experimental data. As a caution for performance prediction in structural dynamics, while acceptable reconciliation of natural frequencies from FE model and measurements was achievable, reconciling simulated effects of changing mass with observed behaviour has not been straightforward due to the complexity of the retrofitted suspension bridge structure studied.