The mathematical model structure of the sustainability of transport flow in various road network sections under the action of external disturbances is justified. The proposed mathematical model considers process development dynamics. In addition to speed and density gradients of traffic flows, the dynamic properties of vehicles, the multilane of highways, and the time of delays at pedestrian crossings and traffic lights are considered. According to this, a third-order differential equation describes transport stream dynamic properties. Dependences are obtained to determine gain factors and time constants included in the differential equation. In this study, the parameters are justified: the input impact on the system and effects on the sustainability of the transport stream—these are density and speed rate gradients. The parameters are justified in characterizing transport stream responses to disturbances. These are time constants. According to them, all links included in the model are inertial. The term traffic flow robustness is defined, and a criterion for determining the sustainability boundary is designed. Traffic flow robustness is a dimensionless quantity that characterizes the stable movement range of vehicles on highway sections, considering its infrastructure, density, and traffic intensity without delays and traffic jams. It has been shown that the robustness criterion can be applied when analyzing a road network for delays during traffic and traffic jams.