Traffic congestion continues to have a serious impact on the productivity of the nation's transportation system. Signalized service interchanges in urban areas serve as a critical connection between freeway and arterial surface street systems. These facilities often experience serious congestion problems because of high traffic demands exceeding capacity and because of their common, closely spaced ramp terminals. Moreover, service interchanges are usually a part of a signalized cross arterial and may also have short spacings to adjacent signalized intersections. Major deficiencies exist in the analysis technology and software tools available for analyzing traffic congestion problems at oversaturated signalized interchanges and closely spaced arterial systems. Results are provided of an investigation into the nature of oversaturated systems and also undersaturated systems that may become congested (oversaturated) because of poor signal timing and deficient spacing between the signalized intersections. Although the initial research focus was on signalized service interchanges based on calibrated NETSIM simulations, the research results are also applicable to closely spaced signalized urban arterials. The research shows that traffic congestion is complex, but it can be characterized and modeled. An upper bound on signal control delay is provided for oversaturated arterial operations.