The Signal Control Network as the Urban Mobility Nerve Center

Whereas the freeway system is characterized by interstate and inter-regional continuous flow, even at the junction of major highways, the urban context consisting of major and minor collectors and some minor arterials is characterized by a network of signalized intersections whose performance is governed primarily by the performance of the intersection control algorithms to limit delay and minimize energy. Whereas low volume intersections can be regulated with stop and yield signage, or more recently roundabouts, once volumes exceed defined thresholds, signalization is warranted to de-conflict traffic movement safely, and promote efficient progression. Signal control originated with pre-timed sequences implemented with electro-mechanical controllers, a few of which are still in operation today. However, most control has been turned over to software control within signal cabinets sometimes operating with simple pre-timed sequences for particular time of day and day of week, and sometimes in demand responsive mode, altering timing in real-time in response to feedback from infrastructure sensors, primarily inductive loops (and technologies developed to directly replace inductive loops, while emulating their signals). Even modern adaptive control systems are fundamentally limited by the information provided to them from sensors, which typically are inductive loops or one of its many technology emulators. With more traditional fixed-timing approaches for corridors, sometimes sensors placed on side streets limit green time to when a vehicle is detected. Sometimes sensors are placed on the dominant through-street movement approximately 400 feet in advance of the signal, extending green times when more vehicles are sensed, or shortening green times when sensors detect the queue has been expended and no other vehicles are approaching the intersection. In any of these scenarios, the control algorithm is limited by the information that sensors (inductive loops and equivalent technologies) provide to the controller. Such information is at best incomplete and provides only partial observability. Even video sensors which are becoming more and more prevalent over the past decade, are engineered to mimic the equivalent information that an inductive loop typically provides a controller.