I live in a major city, and there are "traffic websites" that list the speeds of vehicles all along the major highways and streets. They know to the mph, and seem to change by the second, in real-time.

My car's GPS device shows traffic and computes my route based on those same things, and tells me where and how long any delays are.

How do they know the speed of traffic at so many places along so many roads and highways? Are there sensors in the pavement? Radar? Satellite technology? Scientology? Internet magicks?