How to coordinate the intersections of a district or a city?
Usually, a signalized intersection in town is not isolated, and it may be wise to consider neighboring junctions. Among the control strategies of signalized junctions, coordination by ''green wave'' is particularly applicable in urban and dense grid networks with only small distances between intersections. These strategies often prove difficult to implement, especially when trying to balance road safety with the free flow of traffic.
Setting up a green wave consists of calculating the best possible synchronization of the traffic lights so that a stream of vehicles, travelling at a chosen speed in one direction, systematically meets green lights.
The principle is to have all intersections operating with a common cycle time and time differences for each traffic phase based on a common time.
- Ensure smooth traffic flow, avoiding frequent stops and starts, thus minimizing air and sound pollution.
- Avoid blocking traffic in a grid network with short distances between intersections while maintaining the consistency of flow and therefore the movement of traffic.
Coordination by cabling
The cables are arranged to connect the controllers together. One controller serves as an orchestra conductor to synchronize the others. We're talking about UNITOP. This cumbersome technique requiring cables is less and less used, at least on new installations, except for sites where optical fiber cable is used which can be shared with other users.
Coordination by time
Each controller must have regular time resetting (performed by a receiver clock; France Inter, Frankfurt, GPS, Pulsatis or others …). Without this there would be no assurance that the controllers share identical time due to different aberrations of their internal clocks.
From the current time (hh:mm:ss) common to all, a Time Base Sync (TBS) is calculated which is common to all controllers. This is the time base that is used to control the phases.
Coordination by a central station
Each controller needs to have a permanent connection to a PC (wired network or 3G internet networks). Each controller leaves the current phase upon receipt of the corresponding order issued by the central station (''go'').
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