The countdown counters are about the same size as one of the lights whereas to be able to comfortably see the counter from a moving vehicle from at least 100 meters, the numbers on the counter should be humongous. After all, the main idea behind this is knowing beforehand (long enough beforehand) what the light will turn into and when so that you can accelerate just enough to make it through just before the green ends, or decelerate enough to make it to the intersection just as the green comes on so that there's no need for braking, stopping and starting again.