So how does Google Traffic work?

I understand how it would work in major metro areas on interstate highways - they can tap into the state transportation department’s already existing system of cameras and mapping systems

but apparently google traffic works on city streets (at least in chicago, and they admittedly are arterial) and it works in semi-rural areas (20 miles from a city with a metro area of 300k, tops)

any thoughts on how this works?

Most Sprint phones come with Sprint GPS. When you fire it up, it asks if you want to anonimously report info back to Google.

What it does is report your speed over Sprint’s data connection. So, if you are on an Interstate, and you are doing 20mph, there must be heavy traffic.

So that’s one way.

that was actually one of my thoughts - i didn’t think about smartphones, but i thought maybe those cars with factory-designed GPS systems that can also receive traffic reports also transmit back your car’s telemetry.

thanks for your observation about Sprint - did not know that one.

The answer given is correct. Here’s a post on the Official Google Blog about it when they expanded traffic information to surface streets.