Ever wonder how those digital signs that say how long it’ll take to get somewhere are so accurate?

Devices along the road pick up “ping pulses” emitted by cellphones in passing vehicles that are in search mode and estimate the time needed for those vehicles to get to the destination identified on the sign.

It may come as a surprise — it was astonishing to me, no tech wizard — but it turns out that all of us with phones or Bluetooth devices in our cars are providing the information on which the estimates are based to “variable message signs.”

Mr. Pip Wedge emailed me about one such sign that conveys the estimated time to get to Eglinton Ave., where construction of the Metrolinx Crosstown LRT has for years tangled traffic.

“Each time I drive south on Yonge from the 401, I see an electronic sign saying ‘10 minutes to Eglinton,’ (or nine or 13 or whatever),” said Wedge.

“It’s invariably pretty accurate, and my inquiring mind always says, but how do they know, and who are they? Any idea?”

As it turns out, “they” are us. Or those of us with cellphones in our cars.

And it’s a reminder of how closely we’re tracked by our phones, often when we have no idea of that we’re being monitored.

I thought the signs belonged to the city, so I posed Wedge’s question to Eric Holmes, a spokesperson for transportation services. He said the signs, on streets like Yonge and Bayview Ave., are provided by Metrolinx, as a courtesy to drivers heading in the direction of Crosstown LRT construction.

I asked Anne Marie Aikins, a spokesperson for Metrolinx, why the signs are so uncannily accurate. She said she didn’t know the signs were Metrolinx’s, but would find out how they worked.

She soon replied that the signs are provided by a contractor and are part of a traffic management plan that co-ordinates with travel-time data gathered by the city.

So back to Holmes to ask how the data is gathered. He directed me to a page on the city website that lays out how cellphones in passing vehicles are used to figure out travel time to Eglinton or other destinations on signs on the Don Valley Parkway, Gardiner Expressway and Lake Shore Blvd.

It said “the system uses a combination of Bluetooth and Wi-Fi readers to detect vehicles anonymously on the road that are either equipped with such enabled devices or have drivers or passengers with enabled cellphones. The system takes an average of the readings and then reports a representative travel time to be displayed on the variable message signs.”

It went on to explain that “Bluetooth/Wi-Fi readers are small, low-powered devices that have the ability to anonymously pick up Wi-Fi or Bluetooth enabled devices such as those found in cellphones, cars, or other electronic devices such as printers, faxes, etc.

“When two such readers are placed in sequence along a particular segment of a road, readings can be matched up between units to estimate the average travel time experienced along the route segment.

“When a Bluetooth or Wi-Fi device is in ‘search’ mode for a connection, the device sends out ping pulses. The Bluetooth/Wi-Fi readers pick up on the pulses, encrypt the device MAC ID and stores the information until a match is found at a downstream location.

“Once the encrypted device IDs are paired, the ID information is deleted and a record is generated based on the time/stamp and segment travel time,” adding that “the system is ... updating the signs every minute.”

A system that gathers information from thousands of people who don’t know they’re being tracked raises privacy concerns. But the city says measures are in place to ensure the anonymity of those from whom the data is gathered. “The device MAC IDs are completely anonymous and not traceable as there is no database linking device MAC IDs to specific users. Moreover, the IDs are not stored in the system records and even while the system is looking for a match, the stored IDs are encrypted.

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“There are no specific link travel times stored for specific users (i.e. the system cannot be used to identify a specific user’s speed or travel time). Hence, there is no way for anyone to be capable of working backwards through the system to identify specific users and times as the information simply isn’t there.

“The anonymous nature of this technology has made it a very common approach to detecting general traffic conditions by road authorities throughout North America and other parts of the world.”

So now you know how Big Brother knows. You’re being watched.

What's broken in your neighbourhood? Wherever you are in Greater Toronto, we want to know. Email jlakey@thestar.ca or follow @TOStarFixer on Twitter

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