We propose a methodology for identifying queue spillovers in city street networks with signalized intersections using data from conventional surveillance systems, such as counts and occupancy from loop detectors. The key idea of the proposed methodology is that when spillovers from a downstream link block vehicle departures from the upstream signal line, queues discharge at rates smaller than the saturation flow. The application of the methodology on an arterial site and the comparison with field data show that it consistently identifies spillovers in urban networks with signal-controlled intersections. The method is extended to account for the variations in vehicle lengths. We also investigate the significant effect of spillovers in congestion and show that a macroscopic diagram that connects spillovers with vehicle density exists in large-scale congested urban networks.