> I'm not 100% sure, but I was told that the data comes from a company that
> monitors the flow of cell phones on highways. When the cell phones all
> slow down, they know the road is congested.
They use a bunch of techniques. There are loop sensor in some places,
there are "speed cameras", some places use electronic toll tag readers.
Cell phones are used in some places, but they're the least accurate;
and there are also some historical statistics to fall back on (I
built the system that generates those - and I'm really glad that it's
not my problem any more).
The downside of the Google Maps approach is that it's useless when
you are out of cell phone range.
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