Why does the signal light blink in the dashcam video?

When viewing dashcam video, not just mirror-type, the LED traffic lights may appear to flicker and blink. Why is this?

Actually, the light of the LED signal itself is originally blinking at high speed. The human eye cannot perceive such high-speed blinking because of the afterimage, but the camera records it. The same is true for all LED lights, not just traffic lights.

However, some products do not appear to have this flashing light. Rather, many cheap products made in China are like this.
This looks better at first glance, but in fact, there is a fatal flaw here. Especially in western Japan, the light of the signal disappears completely for a few seconds at a certain cycle.
This is a fatal flaw for a drive recorder, because during this period, it is impossible to tell whether the signal was red or green.

The reason for this is that LED signals in western Japan flash at a rate of 120 times per second. The general video recording format is about 30 fps (29.97 fps to be more precise), and 120 is a multiple of 30 fps, so for a few seconds or so when the timing is just right, the signal appears as if nothing is flashing.
(Conversely, because of the multiple, the timing of the flashes are aligned, so there is seemingly no flickering or flashing).

In order to prevent this situation, drive recorder products from Japanese manufacturers generally set the video recording speed to 27.5 fps. 25 fps would now be a multiple of the 100 flashes per second in eastern Japan, so by setting the speed to 27.5 fps, which is right in the middle of the speed range, it is possible to see both in western and eastern Japan. This problem of loss of LED signal light can be solved for both West and East Japan.

However, the price to pay for solving this problem is that the LED signal will now appear to flicker at high speed. As far as which one to choose, 27.5 fps is generally used, since it is a drive recorder, and correct recording is considered to be the priority.

Note that this LED signal flickering often does not occur on cloudy days or at night. This is because the camera's shutter speed is longer in dark environments, so the high-speed blinking of the LEDs cannot be recorded in the recorded video, just like the afterimage of the naked eye.



As a side note, this issue is not so much of a problem in other countries, where the LED blinking speed is linked to the power frequency, which in many countries other than Japan is 50hz, the same frequency used in eastern Japan, and this frequency is usually the same within the same country. Few countries have both 50hz and 60hz within the same country like Japan does (this seems to be a historical fact from the Meiji era).