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Structural damage on bridges presents a hazard to public safety and can lead to fatalities. This article contributes to the development of an alternative monitoring system for civil structures, based on passive measurements of seismic elastic waves. Cross-correlations of traffic noise recorded at geophone receiver pairs were found to be sufficiently stable for comparison and sensitive to velocity changes in the medium. As such velocity variations could be caused by damage, their detection would be valuable in structural health monitoring systems. A method, originally introduced for seismological applications and named Passive Image Interferometry, was used to quantify small velocity fluctuations in the medium and thereby observe structural changes. Evaluation of more than 2 months of continuous geophone recordings at a reinforced concrete bridge yielded velocity variations Δv/v in the range of -1.5% to +2.1%. The observed fluctuations correlate with associated temperature time series with a striking resemblance which is remarkable for two completely independent data sets. Using a linear regression approach, a relationship between temperature and velocity variations of on average 0.064% °C(-1) can be identified. This value corresponds well to other studies on concrete structures.