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Insight into how traffic signals work around the city

The City of Ankeny Public Works Department operates and maintains a system of 46 traffic signals throughout the city.  Each signal operates partially by independent timing and partially by coordination with the adjacent signals along a given street. Our signals are connected by fiber optic cables, which allow the signals to be sequenced and timed with each other to allow progression during busy traffic times.

Signal Improvements

The growth of our city has facilitated the expansion of our traffic signal system. In recent history, the city has added at least one traffic signal per year. In addition to new signals, the city has recently upgraded all of our signals by:

  • Changing out the standard signal bulbs with LED bulbs. The LED bulbs put out a different, somewhat brighter light for each of the red, yellow or green indications. The LED bulbs also are much more energy efficient (they use a fraction of the electricity of the old style of bulbs) and last much longer before burning out.
  • Changing out the pedestrian signal heads from the old standard "walk / don't walk" indications to the new style "countdown" indications. This is a safety improvement for all of our signals, as the new countdown style provides a definite indication to pedestrians so they know how long they have to cross the street.

Signal Operations

Each traffic signal is operated by an independent controller located next to the intersection. The controller changes the phases for traffic approaching from each direction using an internal timer and actual detections of the number of cars approaching from each direction. Each signal controller has a timing plan for the number of seconds each phase will stay on, given the number of cars waiting on the given approach.  

Cameras and Loops

Each signal controller "sees" the approaching cars in each direction by either cameras or induction loops. About 2/3 of our signals have cameras mounted on the mast arms or in other locations such as on a nearby light pole. The cameras are the modern way to detect approaching vehicles, and we have found them to be generally reliable and accurate. Please note that our cameras are for vehicle detection only. The City of Ankeny does not use cameras for traffic enforcement.

On the remaining signals, the vehicle detection is by induction loops, which are wires buried in the street pavement at the intersection. With the loops, a car is detected when it drives over the loop and interrupts a magnetic field within the loop. The loop detectors are an older style of detection than the cameras, and are generally not as reliable. As our budget allows, we are removing the loops and replacing them with cameras.

Battery Backup

The signals at our busier intersections are equipped with battery backups, which allow a signal to operate for at least 1-2 hours without power. After that, the signal would go into an all-way flash mode.

Signal Preemption

The signals on our busier corridors such as Highway 69 and First Street are equipped with emergency vehicle preemption equipment. These signals are set up to automatically turn green for fire trucks and ambulances that are travelling to an emergency call. As soon as the emergency vehicle passes, the signal returns to normal operation.

Reporting Traffic Signal Issues / Problems

If you have a concern with the operation of one of the city's traffic signals, please report the problem to the Public Works department by filling out our online traffic signal report form.

Knowing all of the specific details such as the time of day, conditions, traffic, etc. really helps us to trouble-shoot your problem.

On past requests, we have found that many of the problems can be traced to faulty vehicle detection (i.e. the controller is not "seeing" your car). The loops in the pavement get worn and can fail, or the cameras can get misaligned by strong winds, or they can miss vehicles due to precipitation, glare, etc. These problems are usually easily remedied by our signal technicians.

Other requests we generally receive focus on the timing of the signals, and these are more difficult to solve. If the problem is related to a red light staying on too long, or if a certain direction does not get enough green time, the problem may be inherent to the timing plan for the signal. While our staff can make some adjustments to the red and green times that may help address these timing issues, the amount of cross traffic can limit what we can do. This is especially true at peak commute times when traffic from all directions at an intersection can be heavy.

From time to time, public works also receives requests to install new signals at an intersection. To see if a signal is needed, we count vehicles over a 48-hour period and check the traffic volumes against a set of standard warrants. When enough of the warrants are met, the intersection is deemed warranted for signals. If the intersection is on a state highway within Ankeny, the Iowa DOT must approve of the signal installation as well. At that point, a signal installation project could be included in our capital improvements plan, to establish the future funding for the signals. A new set of traffic signals generally cost at least $250,000.

Last updated: 2/7/2012 12:06:20 PM