Police spotting (junction watch)

we're on a bicycle, we're not going at a breaknecking speed to not tell what's the symbol behind the red line.

Yes, but some people have poorer eyesight, are hard of thinking, etc. etc. Same reasoning as website accessibility - you might be able to read that small, light grey type on your well-calibrated LCD monitor but to a visually impaired pensioner on an old, small, fuzzy CRT screen it might be invisible. It ought to work for the largest possible percentile.