A microwave interferometer has been employed to remotely detect the oscillations induced by vehicular traffic on the dome of the Baptistery of S. Giovanni in Firenze (Italy). The measurement has been carried out to assess the structure oscillations reduction after the ordinance issued by the Major of Florence forbidding all kind of vehicular traffic in the square around the Baptistery starting from October 25, 2009. A simplified preliminary finite element model of the structure, using average literature mechanical parameters, has been used to determine the best instrument installation position and the structure area to be surveyed. Subsequently, the dome oscillations have been recorded for two consecutive weeks starting one week before the traffic block. A spectral analysis on the acquired data has been then performed and a more complex FEM model has been built to interpret the physical meaning of the acquired data. The material mechanical parameters have been finely tuned to reproduce the lowest spectral component found which was very close to the main mode oscillation frequency predicted by the simplified model. The obtained values of mechanical parameters are reasonable for the monument materials and slightly different from average. Measurement results indicate that the traffic block caused a reduction of the daytime maximum oscillations power of around 50÷60% and a decrease by approximately 33% has been observed in the peak component particle velocity. The residual measured oscillations are attributable to human activities around the square.