Message boards on some Valley freeways display how many minutes it will take for drivers to reach specific points ahead, but there’s a limit to how much warning they can offer.

Because the signs are only on the freeway, drivers about to enter the highway have no way of knowing if they can expect smooth sailing or frustrating gridlock.

Chandler has solved that problem at three of its busiest roads by posting actual travel times on digital message boards that are posted well ahead of freeway on-ramps. The signs are updated every minute, said city transportation engineer Mike Mah.

That gives drivers a chance to resign themselves to a long commute — or to quickly plot another route.

“They know what the travel time is every day because they see it every day,” Mah said. “When the number changes, they’ll know that there is more or less congestion than what they typically see.”

The signs rely on data from the Arizona Department of Transportation’s network of sensors on the freeway. But in what Chandler says is a first in the U.S., the city uses Bluetooth detectors to track the movement of cell phones in passing vehicles. A computer adds the freeway and surface street travel time to display on the signs. Each message board posts travel times to two points on the freeway.

“We update the travel times every minute so all of the information that you see on the signs are in real time, within one minute of being collected,” Mah said.

The city has seven sensors mounted on poles that pick up signals from wireless devices. About 8 to 10 percent of passing vehicles have at least one Bluetooth device, Mah said.

The city developed the technology with Phoenix-based Oz Engineering. Oz has installed buried sensors in freeways that detect how fast cars are traveling. Those sensors wouldn’t provide valuable information on surface streets because they don’t take into account how much time vehicles spend at red signals.

But tracking Bluetooth offers another challenge, said Tomas Guerra, an intelligent transportation systems manager at Oz.

Some mobile devices are in cars on the road, while other signals are coming from bicyclists, nearby homes or side roads. To calculate accurate travel times, a computer has to sort out what signals are likely on the road and which ones aren’t.

“Those are the things that make it more of an art than a science,” Guerra said.

Chandler has had the system in place since June, and the Intelligent Transportation Society of Arizona has named it the best state project of the year.

The $400,000 cost was funded by a federal grant. Signs operate from 6 a.m. to 7 p.m. Monday through Saturday.

One of the signs is placed for northbound Price Road traffic approaching the Loop 101-202 interchange. The others are for northbound Arizona Avenue vehicles approaching Loop 202 and for westbound Chandler Boulevard traffic approaching Loop 101.

The Bluetooth detectors collect media access control addresses that are unique to each electronic device. That doesn’t include a phone number or who owns the device, Mah said.

“It is anonymous and we don’t have any information on who may be driving down the road at any one time,” Mah said.

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