Document Type

Journal Article

Publication Date


Subject Area

operations - traffic, infrastructure - bus/tram priority, infrastructure - traffic signals, mode - rail


Traffic signal priority systems, Traffic signal preemption, Traffic signal controllers, Short circuits, Railroad grade crossings, Preemption (Traffic signals), Level crossings, Interconnection (Electric power), Highway railroad grade crossings, Highway rail intersections, Highway grades, Grades (Roads), Grade crossings, Electronic traffic controls, Controllers (Traffic signal control equipment), Automatic train supervision


This feature examines the basic interconnection circuit, as defined in the Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD), and explores some of its limitations. As defined by the Technical Working Group, interconnection is the electrical connection between the railroad active warning system and the traffic signal controller assembly for the purpose of preemption. Preemption of traffic signals requires an electrical circuit between the control relay of the grade crossing warning system and the traffic controller. The circuit shall be of the closed circuit principle; that is, the traffic signal controller is normally energized and the circuit is wired through a closed contact of the energized control relay of the grade crossing warning system. This normally energized circuit arrangement is considered fail-safe: if there is a break in either or both wires/cables of the circuit, the traffic-signal controller unit would respond as if a train is approaching, clearing motor vehicles off the tracks, even though a train may indeed not be approaching. Supervising the interconnection circuit is intended to mitigate these problems. Supervised interconnection circuits notify the traffic-signal controller if there is a problem with the physical wires or cables that run between the railroad grade-crossing warning equipment and traffic-signal equipment. Potential problems include a utility or contractor inadvertently digging up and severing the wire/cable or loose wire/cable connections in one or both of the cabinets. Another potential problem is a short in the interconnection circuit. A possible solution is to place the AC or DC control voltage source at the other end of the circuit, away from the control relays.