Sensors on I-5 provide real-time traffic data

By Bill Sheets

Herald Columnist

Larry Longley of Stanwood writes: I have recently noticed that on I-5 between Marysville and Arlington, there are new sets of two (and sometimes three) large circles that are cut into each freeway lane. I am assuming that these are some type of traffic counter device.

Why are they in groups of two or three, together in each lane? Sometimes there are as many as three or four such sets of circles between on and off ramps.

I wonder why there needs to be so many of these sets between on and off ramps, when there can be no change in the number of cars on the road in that section.

I wonder why the need for these north of Marysville where the traffic is usually not congested, and almost always flows freely in this area.

And what is the cost of installation and continued monitoring?

Bronlea Mishler, spokeswoman for the state Department of Transportation, responds: These sensors are called vehicle detection loops. They are used to gather traffic data, including speed and volume. Similar loops are installed on I-5 through Everett and into Seattle. The loops monitor the flow of traffic and provide that information in real time to the Seattle traffic website at

The traffic flow map uses different colors to show different levels of congestion. Green is a free flow of traffic, whereas red or black indicate significant congestion or stop-and-go traffic. Areas in grey are where we don’t have any vehicle detection loops to gather traffic data.

Locations where you see two or three loops in a row are used to determine speed based on the time takes for a vehicle axel to travel from one loop to the next.

Even though the number of vehicles on the road doesn’t change on those long stretches between on- and off-ramps, we still need to monitor the flow of traffic. We space the vehicle detection loops out uniformly throughout a stretch of road. In downtown Seattle, the sensors are spaced between a half-mile and mile apart.

In Marysville, the sensors are placed about a mile apart. This allows us to more accurately monitor the volume and speed of traffic. If the sensors were placed several miles apart, and some sort of incident stopped traffic in between the sensors, we wouldn’t be able to accurately collect that traffic data and provide it on our website.

This area may not typically be congested, but during busy holiday weekends or in the event of a serious collision on I-5, the new detection loops will help us better monitor the flow of traffic and provide that information to drivers on our Seattle traffic page.

The flow map on our Seattle traffic page provides drivers with information about traffic conditions between the King-Pierce county line and Arlington. This allows drivers to plan their routes ahead of time to avoid congestion and delay. If you look at the map, you’ll notice that traffic condition information is hit-and-miss just north of Everett, even though the map stretches north to Arlington.

These new vehicle detection loops will allow us to add information to the map to give drivers better real-time traffic information.

The cost to install the detection loops, electronic messaging signs and traffic cameras between Marysville and Arlington, as well as the fiber optic cables to support this infrastructure, was $2.5 million.

This work was done as part of the Marysville median barrier project at projects/I5/marysville medianbarrier.

This funding came from the American Recovery and Reinvestment Act. We currently have staff who monitor cameras and traffic volume on highways between Auburn and Everett, so monitoring this new traffic data will not increase costs.

Additionally, these new detection loops and traffic cameras are expected to last at least 20 years and will require only minimal maintenance.

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