How do traffic department personnel detect a car going over speed limit ? [I Obey the Speed Limit]

What sensors do they use ?

And if anybody knows what data the sensors gather and how they calculate instantaneous speed, that would be great too ?

Or do they use a pair of sensor located close to each other ? That would make it easier. IDK

This is NOT a homework question, BTW

And most importantly, can bluetooth sensors be used to do that?

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Roadsterrider · 56-60, M

It depends on the agency, some use radar, some use lasers. Some use time, ever drive across a white line painted on the road and then there would be another line a few 100 meters down the road? An officer in an aircraft will start a stopwatch when a car crosses the first line and again when it goes across the 2nd line. Doing the speed limit, it takes 4 seconds to travel between the lines, if you travel between them in 3 seconds, you are exceeding the speed limit. Laser and radar both work the same way, a beam of radar or light, at a certain frequency, goes out and bounces back from the target, the changing frequency tell how fast the target is approaching.

MasterofNone · 26-30, M

@Roadsterrider Thanks. Very informative. Some use ANPR cameras. They take picture of a vehicle and see its number plate on one location. Then they have another camera somewhere else (I don't think they are as close as somewhere just 4 seconds away from the first). And they take the time it took for the car to reach from the first camera point to the next. And whether the average speed was more than the speed limit. However, is it a good enough way to detect speed ? Because it is just measuring average speed and not instantaneous speed at every point? Maybe the vehicle was too fast as well as too slow in some places making the average acceptable ?

Roadsterrider · 56-60, M

@MasterofNone The only cameras I have dealt with are connected to a laser or radar unit, it doesn't take a picture unless you are speeding.

MasterofNone · 26-30, M

@Roadsterrider That would make sense.

Have you dealt with any bluetooth sensors in this domain ?

Roadsterrider · 56-60, M

@MasterofNone No, I have no idea how that would be used to determine speed with the exception of some kind of GPS input or calculation.

MasterofNone · 26-30, M

@Roadsterrider What I know about bluetooth sensors is that they take in data from the bluetooth devices inside a car to identify it or the person inside it. Then another bluetooth sensor on another location will do that again. The time it took the car to travel from one bluetooth sensor to another could be used to calculate average speed. But calculating average speed instead of instantaneous doesn't sound very useful.

Roadsterrider · 56-60, M

@MasterofNone I don't care if they monitor speed or not, my biggest problem with blue tooth is that it is basically a surveillance system we are almost forced to use. Some states have talked about a "use tax" for highways based on how much you drive. The data is there, it just has to be sorted. The government doesn't need to know where I am 24-7 and they have no right to collect that data. Please excuse me while I go put my tinfoil hat back on.