Two-layer hierarchical control for large-scale urban traffic networks

Many efforts have been carried out to optimize the traffic signal settings in cities. Nevertheless, state-of-the-art and -practice strategies cannot deal efficiently with oversaturated conditions (i.e. queue spillbacks and partial gridlocks), as they are either based on application-specific heuristics or they fail to replicate accurately the propagation of congestion. An alternative approach for real-time network-wide control is the perimeter flow control (or gating). This can be viewed as an upper-level control layer, and be combined with other strategies (e.g. local or coordinated regulators) in a hierarchical control framework. In the current work, a recently developed perimeter control regulator is utilized for the upper-level layer. Another lower-level control layer utilizes the max-pressure regulator, which constitutes a local feedback control law, applied in coupled intersections, in a distributed systems-of-systems (SoS) concept. Different approaches are discussed about the design of the hierarchical structure of SoS and a traffic microsimulation tool is used to assess the impact of each approach to the overall traffic conditions. Preliminary results show that integrating a network-level approach within a local adaptive framework can significantly improve the system performance when spillback phenomena occur (a common feature of city centres with short links).