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How Real-Time Traffic GPS Works

How your GPS navigator or cell phone knows when trouble is up ahead.


by Todd Haselton on March 4, 2009

traffic_sh.jpgYou’re driving down the highway when your GPS navigator tells you there’s a traffic jam on the way to your destination. No problem: You click the reroute button on the screen and follow directions to bypass the mess. It sounds simple enough, but a lot of work goes on behind those red, yellow, and green traffic-status lines. We spoke with the New York State Department of Transportation, as well as Scott Sedlik, vice president of product marketing, and Jason Kheriaty, technical accounts manager at Inrix to find out how it all works.

Q: Who collects live traffic info?

A: Traffic data is collected along 6,000 miles of U.S. highways by the Department of Transportation. There are also private aggregator companies such as Inrix, which provides data to 70 percent of all live-traffic subscribers. Partners include the biggest names in GPS, including Garmin, Mio, Navigon, TomTom, and even AT&T and Sprint (with certain cell phone models).

Q: How does the Department of Transportation gather real-time traffic data?

A: “We use everything from modern fiber-optic cables embedded in the pavement to sensor boxes along the highways that need to be manually checked by employees walking along the road on a regular basis,” explained Carol Breen, spokesperson for the New York State Department of Transportation. “Some of these sensors are electronic, and others are solar powered.” The DOT also uses loop sensors that can distinguish between cars and trucks, depending on how fast the rear wheels follow the front tires over the sensor. Police officers and DOT vehicles provide additional data.

Q: How do traffic aggregators gather data?

A: Inrix collects data from hundreds of sources, over 100,000 miles, including those gathered by the DOTs. Additionally, agreements are in place with fleet-management companies, such as airport shuttle services, taxis, gas utility, and home-repair companies, as well as long-haul trucking firms. These fleets all have GPS probes in every vehicle to keep track of the car or truck’s location.

Most truckers send data between 6 a.m. and 8 p.m., while taxis and other public-service vehicles broadcast conditions around the clock. Inrix passes off routing and traffic information to the fleets—which manage nearly a million vehicles in total—to help them avoid traffic. In exchange, their vehicle GPS probes relay the road conditions back to Inrix. If needed, Inrix will pay some fleets for traffic data as part of a monthly or yearly contract. 

The company also uses cell phone data from GPS-enabled devices. When a passenger in a car requests a GPS position from a phone, the speed and location of the phone is sent to Inrix.

Inrix also has a partnership with the Clear Channel Total Traffic Network. Clear Channel keeps track of incidents, construction, and accidents using helicopters, as well as reporters stationed on the ground and around the country. Both companies share and resell each other’s traffic data.

Q: How are traffic conditions delivered to my GPS device or cell phone?

A: Data can be sent to your cell phone or personal navigation device in one of four ways: Radio Data System–Traffic Message Channel (RDS-TMC), General Packet Radio Service (GPRS), satellite radio, or MSN Direct.

Clear Channel’s RDS-TMC network is the largest in the United States, and this FM sideband technology is the most popular traffic data delivery method. Information is sent from nearby FM towers.

MSN Direct compresses and decompresses data over FM sidebands. Its feeds also include such information as weather, movie times, and gas prices. This technology is available in select personal navigation devices from Alpine, Avis, Garmin, and Pioneer.

XM/Sirius satellite radio traffic services are used primarily in vehicle-embedded GPS systems and can be found in about 700,000 vehicles nationwide. But GPRS-connected devices allow for the best experience, since they are capable of uploading and downloading personalized data. Inrix powers the traffic data for AT&T and Sprint devices, while Navteq delivers the data to compatible Verizon Wireless phones. Traffic info is sent via nearby cellular towers.

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