E-ZPass Could Kickstart Smart Cities

A small off-white plastic box with two protruding black antennas
Photo: MIT
By using two antennas, the Caroake unit is able to determine in which direction EZPass transponders are signalling from.

Everyone likes the idea of a smart city. Traffic lights would automatically adjust to optimize traffic flow, you could find parking spaces without circling for hours, and enforcing speed limits wouldn’t require a cop on every corner.

The problem is that technologies currently underpinning these systems are often expensive, unreliable, or both. Cameras need pricey computer vision systems to know what they are looking at, and require cleaning, while sensors embedded in the road are too expensive to monitor all but the priciest parking spots. And radars are good at measuring speeds, but poor at specifying individual vehicles.

Now researchers at MIT have designed a smart city system that leverages the windshield tags used to smooth drivers’ passage onto toll roads and bridges. E-ZPass tags in the eastern United States—and similar systems nationwide and around the world—are radio-frequency transponders that transmit a unique signal when queried at a certain frequency. That response is typically picked up by a reader mounted on a gantry over the highway, and the driver’s credit card is automatically charged.

However, Dina Katabi and Omid Abari at MIT realized that anyone could prod a transponder to emit its signal, simply by asking nicely. They have developed a small solar-powered unit called Caroake that can measure the position and speed of up to 20 nearby vehicles that are equipped with E-ZPass transponders. Each Caroake unit currently costs around $100, although that figure would drop significantly if the units where produced in volume, say the researchers.

By putting Caroakes on every urban streetlight, the researchers envisage tracking the occupancy of every parking space, counting how many cars are waiting at every traffic light, and perhaps even catching every speeding vehicle. They are about to deploy six readers on public streets in Cambridge, Massachusetts to learn local traffic and parking patterns.

“There is huge interest both from the city and from E-ZPass, who see this as a mechanism both to expand the functionality of a device that already exists in the vast majority of cars, and to make the city smarter,” says Katabi.

But the pilot involves much more than simply putting normal E-ZPass readers on poles. The highway readers use highly directional antennas to ensure they are querying a single car at a time as it passes beneath them. That technology is both expensive and of limited use in a city, where you want a more holistic view of what is happening along each street.

Instead, the Caroake device activates all E-ZPass tags within a 30-meter range. “In a city, 20 or 30 cars might respond simultaneously, the signals collide and you get a real mess,” says Katabi. Luckily, although all E-ZPass tags are meant to broadcast at 915 megahertz, in practice individual units tend to stray a little to either side of that frequency. The MIT system uses this so-called carrier frequency offset to separate the signals from one another.

Localizing each tag is another challenge. A pair of antennas in the system calculate the signal’s angle of arrival. For stationary cars, that is enough information to locate them in a parking spot. To track moving cars, two or more neighboring readers can work together to triangulate its position. Caroake units form an ad hoc mesh network with each other, and use a Wi-Fi or cellular LTE gateway to push data up to smart city servers.

In tests on MIT’s campus, the Caroake network could determine speed to within 8 percent (about 1.6 to 4.8 kilometers per hour) and location to within 4 degrees, which was accurate enough to place cars in individual parking spots.

Another worry for the researchers was that constantly querying the E-ZPass tags might run down their sealed, non-rechargeable lithium batteries. In normal use, where they might be queried just a few times a day, E-ZPass tags can last a decade. In a smart city, the same tag might be activated a hundred times daily, or more. “We ran experiments and found out that even if you query the transponders every second, their batteries will still run for around 10 years,” says Abari.

But the biggest concern for many drivers is likely to be the privacy and security issues that come from being tracked throughout a city. In 2013, the New York Civil Liberties Union found that city’s Department of Transportation had quietly deployed dozens of standard E-ZPass readers to measure traffic volumes and congestion. The Union called the department’s privacy policies at the time “vague and barebones.”

In their pilot tests, the MIT researchers will measure only the carrier frequency offsets of the transponders and not decode the identifying data bits. “We do not believe that [these] values can be mapped to the owners or used to infer any private information about them,” they wrote.

Of course, if E-ZPass tags were to be used for cash-free parking, they would have to link individual cars to their owners. Dina Katabi thinks that far from infringing people’s privacy, Caroake devices could enable new, convenient business models: “It could be used to pay for other services. For example, I could drive to a restaurant and be charged immediately, or get access to certain roads at certain times, like congestion charging.”

For those services to be available to everyone—and for the smart parking and traffic systems to be most effective—every car will need an active transponder. Adoption rates are creeping up as states like Massachusetts and Pennsylvania sunset cash toll booths in favour of E-ZPass, but they are still at only 80 to 90 percent.

Perhaps a bigger problem is that anyone worried about Caroake’s Big Brother implications (or who just likes to speed) could temporarily pop their tag in the metallic “read prevention” bag that E-ZPass provides with every unit. After all, smart cities will be home to plenty of smart people.

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