The purpose of this page is to provide some basic information on how signals work in the City of Oak Ridge, Tennessee. If you have further questions, feel free to e-mail them to the Department. View our commonly asked questions.
The Old Days
Years ago, traffic signals were controlled by a simple electric clock. These clocks allocated a specific amount of time to each traffic direction, in a specific pattern. The problems with this arrangement are obvious under today’s heavy traffic conditions. The clocks left no room to adjust for peak traffic periods, or unusual conditions.
The next step was to create a clock that operated differently at different times of the day. Still largely mechanical, these devices can still be found in use in some locations. They may have several different patterns for different times of day.
As electronics have influenced everything, so have they changed traffic control. Today, the traffic signal controller is a device that monitors traffic in several directions, and adjusts the signal timing and sequence based on that traffic.
Phases and Control
Before going further, the term “phase” must be defined. When used in traffic control terminology, a phase is a direction of traffic flow that is controlled differently from all other directions. At a simple intersection, with no left turn signals, there might only be two phases: one for Main Street and one for Elm.
As the intersection progresses, more phases are added. If Main Street has left turns, a phase may be allocated to the turning movements. The total number of phases is now 3. If, however, the designer wishes the left turn signals to be able to operate independently, that is North bound traffic on Main could get a turn arrow, while South bound did not, a separate phase is required for each turn movement. This would bring the number of phases to 4.
In a fully actuated and complicated intersection, the number of phases can reach 16. The intersection shown uses 12 phases. These phases include the walk signals. Walk signals must, of course, also be coordinated with the traffic signals.
The situation can become even more complicated if several signals are coordinated. A brief explanation of coordination is provided for you.
It is the responsibility of the traffic signal controller to keep track of the different phases, to make sure that all get the green time they need, and that no conflicting movements get a green signal at the same time.
Safety Measures and Conflicts
Equipment can break down and programming errors do occur. Because of the safety issues involved, signals are equipped with a “conflict monitor.” A conflict monitor is a simple device, completely independent of the controller that watches the signal operate. It does this by monitoring a number of conditions, including the voltage to the individual bulbs in the heads.
If a condition occurs which is not normal (for example opposing greens) the conflict monitor detects the condition and shuts down the intersection. Normally, it places the signal on “flash mode.” The main street is given a flashing yellow, to indicate that the situation is not normal and caution is needed. The secondary street is given a flashing red light that should be treated like a stop sign. For safety reasons, the signal will not normally reset itself. A technician must visit the intersection, determine the problem and reset the controller.
Detecting the Traffic at Signals
Nearly all of the intersections in Oak Ridge use a buried loop in the pavement to detect traffic. These loops create a magnetic field that is disturbed by the magnetic materials in a car passing over it. A special device in the traffic control cabinet monitors the buried loop and reports to the controller when it has been disturbed.
To make a loop, a groove is cut into the pavement. A thin wire is placed in the groove, and the groove is filled with a special material. In some intersections, you can see the groove that contains the loop. In other intersections, the loop has been covered by asphalt.
Loops took the place of the metal traffic plates that were in use at one time. These devices are seldom, if ever, in use today. They exist in the intersection simply because they are so hard to remove.
There are a couple of other devices in use to detect traffic. These are normally used in special circumstances where it would be difficult or impossible to make a loop. The City of Oak Ridge uses microwave detectors in some intersections. These devices resemble a closed circuit TV camera mounted on a pole or signal cross arm.
They function similar to police radar, sensing the presence and direction of traffic. It is possible for extremely slow moving vehicles not to be picked up by the devices, although this is seldom a problem.
Nothing at All
We would not be complete without adding that some movements at some intersections do not have any detection at all. Most commonly, the main street of a main and secondary intersection has no loops. The controller is programmed to always assume there is traffic on the main street, and return green to that movement. Right turn only lanes often do not have detection loops.
Coordination of Traffic Signals
How do you Coordinate Traffic Signals?
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?”
Understanding the term CYCLE TIME
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.
OK, Now what?
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.
45 miles/hour X 5280 ft/mile X 1hour/3600 seconds = 66 ft/second
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.
Why is this HARD?
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.
There are computer programs that solve for the best fit. Generally, they try different combinations to get the best fit based on the total sum of time spent at red lights. Often times, this is to the detriment of specific approaches or streets. If you use those streets, you will not be happy with what is “best” for the traffic.