City of Oak Ridge Electric Department

Coordinated Traffic Signals should turn green just as you arrive at the intersection. This is really nice when you have a string of signals close together. The question is, "how is this done?" or more commonly, "why isn't this done?"

Before we begin, we have to understand a concept called Cycle Time. This is very important in coordinated signals.

Cycle time is the amount of time from the beginning of a red light to the beginning of the next red light. All of the events at a signal happen inside one cycle. Normal cycle times are somewhere between 80 seconds and 120 seconds for most large roads.

The advantage of a short cycle time is that on low traveled streets, the driver does not have to wait a long time for a green light. Unfortunately, it takes time for cars to get going when the light turns green and to reach the traveling speed of the roadway.

The advantage of long cycle times is that they are more efficient in moving cars on the main street.

If you draw a two axis plot, you will see how signal timing works.

On the vertical axis, scale the repetitions of the cycle time. Note that on the attachment, I have settled on a 120 second cycle time. This means that at the end of a cycle, it starts over (at 0).

On the horizontal axis, scale the distance from the first intersection to the next intersection, to the next and so on. In my example, I have scattered the intersections at different distances from one another. This needs to be plotted fairly accurately from the first intersection. I have called my cross streets A,B,C,D, etc.

The next part depends on your design speed limit. If you know that Intersection A is 2800 feet from the starting point, how long will it take your vehicle to get there? To do this requires a little simple math conversion.

2800 feet / 66ft/sec= 42.2 seconds.

OK, plot this point on your graph, and then extend this into a velocity line from the starting point to beyond your area of concern. This is the blue line on the graph.

Looking at the graph now, you can see at what time during the cycle each car will arrive at each intersection. All you have to do now is make sure that any particular signal is green when the velocity line crosses it position.

For each individual intersection the timing will be different. Each must be based on the cycle time, though.

Our example used a one-way street. For a two way street, you repeat the same process for traffic in the other direction, starting from the other end of the section under consideration. Then you try to design green times that fit things as well as possible. Unless you are lucky enough to have good intersection spacing for the speed limit (rare) there will be no perfect fit.

Pedestrians also cause issues.  A pedestrian takes longer to cross the street than a car.  When someone pushes the "ped button," the cycle time for that particular intersection is changed to allow the person time to cross.