Question:  How do the traffic reporters get the current information on freeway congestion for their reports?  

Answer:  There are a few different sources.  The main one is detectors in the pavement that constantly measure speed and send the data to a Traffic Management Center.  In the SF Bay Area Caltrans has a Regional Traffic Management Center in their headquarters building in Oakland.   http://transportationnation.org/2012/04/26/decoding-the-mysteries-of-bay-area-traffic/tmc-main-pc/ 

 The detection loops are embedded in the pavement and look like this.  They are 6' x 6' squares, spaced 10' apart.  The time gap between a car entering the lead detection zone and the second zone is recorded.  Because the distance is fixed, the travel speed of each car can be calculated.  Computer software at the Traffic Management Center "smoothes" the data and puts a summary on a map like the one found at www.511.org.