How do Speed Traps Work?
Let’s first get clearer by what we mean by the term ‘Speed Trap’? There is another name for the same phenomenon, yes, speed trapping is a phenomenon and not a device, and it is called ‘Speed Supervision’. So the question can also be put this way ‘How do devices which measure the speed of a vehicle and send the data to the authorities work?’
The most common of those devices is a gun, a radar gun?
A radar gun basically comprises of a radio transmitter and a receiver. The device emits radio waves at a particular frequency; the waves travel away from the device and hit other objects and are bounced back off them. The radar gun receives those bounced back waves. But in the process the waves undergo a change in frequency. The devices are made sensitive to detect any change in the frequency. And through that change in frequency the device calculates the speed of a moving object.
Why waves undergo a change in frequency?
This phenomenon is called the Doppler Effect. To put it in simple language it goes like this: frequency can be thought of as intensity. In fact, more frequency means more intensity. Intensity of light is the amount of light falling on per unit area, and how powerful a light beam is, in turn, depends upon its frequency. However, frequency decreases with distance.
Now, how would we feel if a speaker turned on at the full volume is placed close to us? The sound would be unbearable, of course, and it is because of the high intensity of the sound. Now, let’s call some friends and push the speaker 100 yards away. The sound would be relatively bearable. Why does that happen, although the speaker is still playing at the maximum volume? It happens because the intensity of sound that is reaching us from that greater distance is lower. A scientist would say that the frequency of a wave decreases with the distance, so the waves that are reaching us now have a lower frequency, are less powerful, and thus have a lower intensity.
If our friends are obliging, we can now ask them to bring back the speaker to us without turning it off. What can we expect when the speaker is being pulled back? We can expect the sound increasing in intensity as the speaker is being brought towards us. Or, in other words, the frequency seems to have changed and increased.
The radio waves (which were emitted by the radar gun) hit a car and are bounced back. There can be two scenarios: the car is coming towards us or the car is going away from us. In both scenarios the principle remains the same. Let’s see what happens in the first scenario:
The car is coming towards us. The frequency of the radio waves that are being bounced back would increase because the car is coming towards us (as happened in case of the speaker). And if the change in frequency can be measured the speed of the object can be found out with the help of a simple mathematical formula:
-Vs, r = velocity (speed) of the moving object (the car) with respect to the observer (the policeman)
c = speed of light (the radio waves) = 300, 000 meter per second
Why are speed traps installed?
An obvious reason is to keep a check on rough drivers and stop them from reaching speeds which may be dangerous to others. Most speed limits have been established after a long study but still have their own critics, for whom it is a pointless exercise.
