The biggest problem I have in this entire discussion, aside from the continued ignorant arrogance of the yellow-flag conspiracy crowd (reading snippets of rules instead of the entire sporting code, questioning the presence of a marshal stand even though the stands location was proven from the FIA previously in the thread, etc), is the assumption that a seasoned professional racecar pilot would not be able to see a green flag waving at 160mph, and YOU couldn’t see it on the low resolution camera shooting in cloud cover , so it must not be possible!
Drivers are trained early on to know where the marshal stations are, and to sight them as they approach. This is covered in even the most basic track day driver’s meeting. Since my first track day, I have known to take note of where the stations are, and most importantly any special circumstances on the day that change the behavior required for certain situations. Besides, with a good wave, the color of the flag being presented can be determined a quarter of a mile away. On approach all it takes is a quick glance to verify. Trust, it’s not that difficult.
Aside from all that, there are MANY racing series where those speeds are reached, and very few of them have electronic signaling. What if the power went out to a station? Flags only need a willing arm, and marshals only need the battery in their radio.