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Dec. 2, 2021

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Clark Asks: How do highway reader boards calculate travel times?

WSDOT system using radar sensors lets drivers see estimated times

By , Columbian environment and transportation reporter
Published:
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A travel-time sign gives traffic in the southbound lanes of Interstate 5 just south of the Clark County Event Center at the Fairgrounds an estimate for traffic down the road. Signs like these use roadway sensors and radar devices to measure the speed and density of passing traffic, and a computer program uses the information to estimate travel times between two points.
A travel-time sign gives traffic in the southbound lanes of Interstate 5 just south of the Clark County Event Center at the Fairgrounds an estimate for traffic down the road. Signs like these use roadway sensors and radar devices to measure the speed and density of passing traffic, and a computer program uses the information to estimate travel times between two points. Amanda Cowan/The Columbian Photo Gallery

Battle Ground’s Jen Kootstra, now retired, doesn’t make the commute south on the interstate highways in the county as much as she did when she was still working as an engineer for HP.

Now, she makes trips south roughly weekly, heading to the 78th Street Heritage Farm, where she’s involved in the Master Gardener program, but she still wonders: How do traffic officials figure out travel time estimates on highway reader boards, such as the one north of Salmon Creek?

Kootstra submitted her question through The Columbian’s Clark Asks website feature, where readers can suggest and vote on questions for further reporting.

Kootstra and her husband, also a former HP engineer, hypothesized that the system used some kind of method that picked up, then extrapolated travel time from, motorists’ cellphone signals.

Her husband has a good track record for such things, she said, “So I’m testing his answer.”

“I’m also just plain curious,” she said.

Washington State Department of Transportation spokeswoman Tamara Greenwell said the agency uses radar sensors or sensors in the pavement to measure the speed of passing vehicles, and density of vehicles on the roadway.

The sensors in the pavement — inductive loops — work like metal detectors, Greenwell said. When a metal object, such as a vehicle, passes over or stops within the sensor’s detection area, it trips the sensor.

Greenwell said these units often need ongoing maintenance, and have to be replaced whenever a road is repaved, and that fixing them requires cutting into the pavement. She said WSDOT is trying to use more radar sensors where feasible.

The sensors feed their data into a computer program that calculates an estimated travel time between two points, and that information is displayed on the readouts. Oregon and Washington share traffic data, so drivers are seeing estimated times calculated in the same way whichever state they’re in.

The agency placed the signs at key spots, so drivers could have time to change their route if needed. The signs in Vancouver were installed in fall 2015.

Kootstra’s husband wasn’t necessarily that far off. Mapping applications such as Waze and Google Maps use, in part, location data shared from users’ smartphones to calculate traffic volume and travel times. They also use information from public transportation agencies.

Columbian environment and transportation reporter
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