I just bought a car with an on-board navigation system includes weather and traffic data overlays, with the information being downloaded through XM radio. Weather info is easily gathered in real time using radar; that’s no mystery. But how about traffic? How is information gathered regarding current traffic speed and density on public roads?
sensors on the road and video monitoring give the data to traffic control centers.
Yep, there’s a network of sensors on the side of the road. I think companies like Navteq, Google, etc have paid money into this system and then license it out to GPS manufacturers and etc.
That’s also why real time traffic is only available in certain areas (typically in the metropolitan areas of decent-sized cities with a few hundred thousand residents or more), because these sensors are not ubiquitous nationwide.
Vehicles are detected in various ways, but the most common is the vehicle detection loop. A loop is (typically) a rectangular object that is about 2 feet by 6 feet and is made of an 18-guage wire that is wrapped around between 3 and 5 times and buried into a saw cut made into the road (and then sealed). These 3-5 turns of wire are connected to something called a vehicle detector by a twisted lead-in that comes off of one of the corners of the loop (usually through a conduit or sealed saw-cut). The detector does two things - first, it provides a small current to the loop, which creates an electro-magnwetic field. The other thing that it does is monitor that field and registers any significant change in the field - typically, the source of the disturbance is a large metal object.
Those detectors can than be hooked up to a variety of different objects which can be used for things like raising barrier gates, activiating a ticket spitter, signalling the start of a traffic timer, sounding an alarm, or being part of a large data gathering system.
Another two sources of data that my provider (TomTom) uses - I suppose the other vendors operate much the same way:
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location data from the other real time traffic information enabled devices in their installed base (they get traffic data via the mobile data network and transmit their location back the same way)
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anonymized mobile phone location data (i.e. how quickly phones move from one cell to another) that they purchase from mobile phone network operators.
So people using modern navigation systems or just driving with their phone switched on all contribute to the aggregate of traffic data available.
Gleanings from the infrastructural communication between cell phone towers gets sold for this purpose too. Traffic jams trigger waves of extra calls from fairly limited geographical areas.
Some areas use fixed radar sensors to measure the speed of traffic flow (just like police radar guns).
As an interesting aside - I was driving on the interstate through downtown a few weeks ago at about 3:00 AM. My car’s navigation system showed it as bright red - the heaviest traffic. The road was, in fact, completely empty, without a single car except for me. Whatever system is in use obviously cannot discriminate between “no cars moving because of heavy traffic” and “no cars moving because there are no cars on the road.”
Regions that have “toll tags” for toll roads or bridges use those devices as well. In the San Francisco Bay area, they read the ID numbers of FasTrak transponders at various points and as any particular tag moves through the highway system, they can aggregate all of them into real-time travel times. eg: 45 minutes from San Jose to Candlestick, 15 minutes from the Maze to the Bay Bridge tollbooth, etc.