The wire loops (in sets of two a few meters apart) are in every stretch of freeway in California. They report the numbers and speed of all vehicles. Have so for decades. This allows the Caltrans operation centers to instantly identify backups due to accidents, etc. Once in a while, the evening news do a story on these operation centers.
So we still don't know what the grey sensors do. Note that these sensors are typically mounted only above the two right lanes.
Check out <http://traffic.maxwell.com> for real-time results of these sensors. You used to be able to get results for individual sensors, but they made the site "easier to use". -- Marshall Marshall Clow Adobe Systems <mailto:mclow@mailhost2.csusm.edu> Freedom isn't being able to do what you like, it's allowing someone else to do or say something you hate and supporting their right to do so.