Common Questions

Most of the traffic signals in Oak Ridge detect traffic with a buried loop. The loop senses changes in the magnetic field it generates when metallic objects pass through. There are several common reasons why a vehicle is not detected.

First, the loops are able to detect only vehicles that pass over the loops. If you are having trouble with a particular signal, be sure that you are stopping with the front of your car at the stop bar. It is often easy to pull forward too far and out of the loop.

Second, some vehicles (particularly bicycles) do not have enough mass to trigger the loops. This is unfortunate, but present technology utilized in the City does not sense bicycles.

Third, the loop may be defective. If you suspect that a loop is not functioning, call the Department and we will have it checked.

The most common reason that a signal goes to a side street when there is no one waiting is that a car has made a "right turn on red." Most signals in the City can detect that the vehicle has turned and will not go to the side street in these conditions. If the signal begins to change to the side street, however, the process must be completed.

If a detection loop has failed, the signal must be set to go to the side street every cycle. If this were not done, the cars entering from the side street would never get a green light. This is a short-term problem that is normally corrected in a few days.

Traffic signals must be programmed to divide a certain length of time (called the cycle length) among all of the active phases at the intersection. If a phase, such as a left turn arrow, is added, one of two things must happen.

First, the cycle length can be extended. Unfortunately, this means that vehicles must wait longer between green lights. As an example, consider that a simple intersection can have a cycle length as little as 82 seconds, while complex intersections have a cycle length of over two minutes.

Second, the length of each movement can be shortened. This limits the number of vehicles that can pass through the intersection per cycle.

Traffic engineers examine these factors when designing a traffic signal. The ultimate goal is to send as many vehicles through the intersection as possible in a given amount of time. In evaluating left turn signals, they must consider the number of cars making left turns, the amount of on-coming traffic and other factors.

Only when all of these factors point to a common solution can a left turn signal be added.

The amount of time you have to wait for a green depends on the cycle time of the intersection. This can be as little as 42 seconds, for simple intersections, to over two minutes for complex intersections with heavy traffic.

Two minutes is a long time to wait at an intersection. If the time exceeds this by much, many drivers will assume that the signal is "stuck" and ignore it.

Traffic signals in Oak Ridge actually are coordinated. There are two computer controlled systems that time the signals on the Oak Ridge Turnpike and Illinois Avenue. The problem, simply stated, is that these streets cannot be perfectly timed.

Designing coordination for a one way street is quite simple. One simply looks at how long it will take a vehicle to travel from one intersection to the next, and times the greens so that they occur at the proper time.

With two way streets, the problem becomes more complex. Timing must be designed so those vehicles traveling in both directions arrive at the intersections at the same time. When one considers traffic from the side streets, and that the intersections are not uniformly spaced, it becomes clear that there are no perfect solutions.

Traffic engineers examine these circumstances very carefully and attempt to design "best-fit" timing. In Oak Ridge, under normal conditions, you may have to stop for two signals on the Oak Ridge Turnpike. If you enter from a side street, there may be more delays as your assimilated into the "platoon" of cars traveling down the roadway as a group.

425-1803.