How do the road signs which give estimated travel times work?

I live in the Bay Area and drive to work on 101. On the freeway, I pass "signs" that give you automated estimates of driving times to various destinations (e.g. 10 minutes to 237, 25 minutes to 280, etc). These signs are updated dynamically, presumably to reflect current traffic conditions, and I have found them to generally be pretty accurate.

But my question is this: does anyone know how they actually work? Is there a traffic helicopter taking pictures and some human expert who then guessimates based on these pictures and updates the sign? Is it automatically counting the number of cars that pass certain checkpoints in a given time period and estimating based on that?

What is actually going on? Does anyone know for sure how these signs work?

I also live in the Bay Area. I remember when, how they estimate driving times, by data gathered by the Fastrak transponder (maybe now the road speed sensors). That is how I remember at that time.

Copy and Paste from the http://www.511.org/

How does the Driving Times feature work?

http://traffic.511.org/faq.asp#f89

They have sensors in the roadbed to monitor road speed and cameras to watch for accidents everything is usually computer controlled.

they monitor the speeds on the road through sensors

Permanent sensors in the roads, traffic cameras, etc.

If you go to http://www.511.org/ you can get the data interactively. The data on the signs comes from the same place.