There is a reason it’s red though, so it stands out. You might not have the time/attention available to clock if the sign has a circle around it if you’re color blind. You see a circle sign with a bike. You have to look extra hard to see there’s another (possibly faint to you) circle on the sign.

That said, I’m not colorblind and forget exactly how that works so maybe the circle actually looks black to them or something.