How do Google traffic maps work? --- Nathan, Mcdonough, Ga.
Google maps offers traffic data in 50 major American cities. You can go to this page to see the cities covered. In a given city, the traffic conditions are shown with a very simple red/yellow/green coloring scheme. If a road has bad traffic, it is covered with a red line.
The drawing of the lines is easy. The hard part is getting the data so that Google knows the traffic conditions. Where does the data come from? According to this page, "the data is aggregated from several sources, including road sensors, as well as car and taxi fleets." Which isn't all that helpful
What does that actually mean? It turns out that a fair amount of effort goes into gathering traffic data. It is relatively easy to put in road sensors, say every mile on major highways, that detect the speed of vehicles going by. But they are expensive. So a company or municipality pays for the sensors, and then it sells the data. There are many different types of sensors available. According to this article, a company like Inrix pays "about $15,000 each to install the necessary 500-600 roadside microwave detectors" and then might charge the state half a million dollars a year to access the data. There are many different buyers for the data, including TV and radio news programs, Google and other mapping companies, GPS companies (which now include traffic data monitoring), cell phone services, government agencies, etc.
Researchers are always looking for less expensive ways to get the data. For example, they have figured out how to gather traffic data from cell phones, as described here. It is also possible to fit taxis and buses with GPS receivers and gather data on their motion, as seen here.