At least through my city. They look just like the red light cameras you see at intersections, except they’re along the highway. I don’t know how widespread this is, and I have not heard any mention of this on the news.
I have no idea where you live, but my guess is that you are seeing traffic cameras.
You know how on the morning radio, they say stuff like “I-66 is jammed to Spring Hill. There’s an accident on route 14 through Winston Falls, so avoid that…blah blah blah”?
That’s how they know.
I guess I need to get with the times. 
I’ve always assumed it was helicopters that tracked road conditions.
They could also be speed cameras.
No, no. The helicopters track you.
[insert Soviet Russia joke here.]
The morning news on TV around here will show live video footage of freeways, and it ain’t from helicopters.
Minnesota’s MNDOT cameras.
Click on one of the black dots on that map, and you can see exactly what traffic looks like on your route into work.
None of this explains the camera my neighbor has pointed at my bedroom window, though.
We have a similar system in Salt Lake City called CommuterLink. The cameras are monitored at a central location, where operators can program electronic signs on the freeways to warn motorists of conditions. They can also direct police and emergency vehicles and adjust timing on traffic lights as needed.
The cameras are available to TV and radio stations for their traffic reporting, and can be viewed online so that you can make adjustments to your trip home from work.
I imagine that they want to know the speed of traffic at the entrance to the intersection. I know they usually have cameras at the beginning and end of tunnels as well to check the traffic flow.
Also to catch you doing any moving violations.
Cameras for traffic control are completely different than those for enforcement. The traffic control cameras are not particularly high-resolution video cameras and do not show enough detail to identify a car/driver for a violation. An enforcement camera is generally a still camera accompanied by a flash unit and can resolve a clear photo of a license plate and possibly the driver.
Where I live there are traffic control cameras everywhere. Enforcement cameras are not as commomn.
Did any of the traffic cameras catch that whoosh? 
Some of them are actually speed sensors, rather than used as cameras. Electronics connected to them compare pictures, second to second, compute average travel speeds, which are then reported up to central information collection sources. Google maps shows traffic conditions based on this data.
Why the “um”?
Today’s news:
Snopes just published a page on this today. According to them, rumors that NY, FL, and CT are installing freeway speeder cameras is false.
Maybe they are going to implement Average Speed Checks, currently used in the UK to manage road works, but will eventually be rolled out on parts of motorways and A roads throughout the UK. They certainly do make people stick to the appropriate limit.
Si