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Effective road traffic assessment and estimation is crucial not only for traffic management applications, but also for long-term trans- portation and, more generally, urban planning. Traditionally, this task has been achieved by using a network of stationary traffic count sensors. These costly and unreliable sensors have been replaced with so-called Probe Vehicle Data (PVD), which relies on sampling individual vehicles in traffic using for example smartphones to assess the overall traffic condition. While PVD provides uniform road network coverage, it does not capture the actual traffic flow. On the other hand, stationary sensors capture the absolute traffic flow only at discrete locations. Furthermore, these sensors are often unreliable; temporary mal- functions create gaps in their time-series of measurements. This work bridges the gap between these two data sources by learning the time-dependent fraction of vehicles captured by GPS-based probe data at discrete stationary sensor locations. We can then account for the gaps of the traffic-loop measurements by using the PVD data to estimate the actual total flow. In this work, we show that the PVD flow capture changes sig- nificantly over time in the Washington DC area. Exploiting this information, we are able to derive tight confidence intervals of the traffic volume for areas with no stationary sensor coverage.