Traffic signal detection devices help optimize traffic flow and reduce delay.

A camera type detector, perched on the signal mast arm, senses changes in pixels when cars enter the intersection.

Nationwide and here in Utah, increased population has given rise to more traffic volume on our roads. While UDOT has been successful at holding the line on traffic delay through new construction, roads are expensive to build. Making our current system work more efficiently is more important now than ever. Using signal detectors helps move traffic efficiently.

Most intersections in Utah have vehicle sensors installed in, under, or above the pavement. UDOT uses variations of four technologies:

Inductive loops are circular wires installed in or under the pavement that generate an electromagnetic field when a small amount of electricity passes through the wires. When a car, truck or motorcycle passes over the loop, the field is disrupted which tells the traffic signal to change.

Magnetometers detect the presence of cars, trucks or motorcycles, by sensing metal.

Video detection “sees” vehicles by using cameras to detect changes in pixels. Some people believe that the cameras, which are usually placed on signal arms, take actual video, catalog images or provide real-time images — this is not the case!

Radar detectors see cars within 500 feet of the intersection.  The radar extends the green light to keep vehicles, especially trucks, out of the decision making zone created when the signal changes from green to yellow — that’s when motorist decide “should I go or stop?”

Each of the technologies have certain advantages and disadvantages; however, all will detect the presence of motorcycles, cars and trucks at intersections.

The signal detection devices are placed at the stop bar and 15 feet back from the stop bar for the through-movements.  The radar zones are larger than inductive loops, magnetometers or video detection and are typically in the range of 65 feet and 500 feet from the intersection.  For lanes that switch from protected turn (a green arrow) to permissive turn (a green ball) detection is placed roughly 50 feet back from the stop bar.

In some cases, UDOT has “Prepare to Stop” signs along high-speed roads where vehicles have not seen a traffic signal within the last mile.  These signs come on a few seconds before the green light changes to yellow to give drivers additional time to stop.

If you see a traffic signal that isn’t working correctly, please call UDOT at 801-887-3700.




  1. Andrew Johnson

    Some intersections run on a timer, via a control box at one of the corners of the intersection. Under normal circumstances, timers work just fine at directing the flow of traffic, but events such as concerts, sports or construction may change these traffic patterns. Operators at the UDOT Traffic Operations Center have control of various State-owned signals, and are able to anticipate and adjust the timing of these signals.

  2. LHD Cars

    It is ok as long as all this new technology works. The trouble is no matter how good the system is it will fail at some time, causing traffic chaos at least, maybe a serious accident.
    For me the best method is having a traffic cop at each junction. Nothing better.

  3. New Sony Cam

    When the green light to keep vehicles, especially trucks, out of the decision making zone created when the signal changes and when red light come so it means we have to stop

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