“LEDs are super fast. If you drive them with DC, they will not flicker. But, LEDs use very low voltage (around 1 volt) and converting 12 or 120 volts to a small DC using resistors or transistor in old-fashioned “linear” regulation wastes most of the power. So, folks usually pulse width modulate (PWM) LEDs. This turns them on and off at a very fast rate. It wastes very little power and allows you to vary the brightness by changing the % of the time it is on. If the on/off frequency is high enough, you can't see any flicker. One issue is that if you move your head quickly, the LED will look like a dotted line. Modern car tail lights have this problem.”
This explains what I suspected was going on in Cadillac tail lights. They flicker in “normal” mode, but when they are lit for braking, the flicker seems to go away.
There is an art work in the Seattle bus tunnel (Assuming it is still there) that uses the phenomenon of “Precise” flicker in LEDs. It uses the concept of a crawling picture in a tall LED array, but it crawls across a single vertical array of LED’s. There are ten or so of these single vertical arrays spread randomly across a large wall. When you just look at one, the lights seem to sort of randomly flash. But when you look away, you can see the shape being formed as if all of the vertical rows for that image are there.
It’s kinda cool.