The sign you are referring to is the "Rider" sign. This sign typically indicates the presence of riders, such as cyclists or horse riders, in the area. It serves as a warning to drivers that they should be vigilant and cautious, as these riders may be using the road or path nearby.

Drivers should lower their speed when they see this sign and be prepared to give way to any cyclists or horse riders. It is important to maintain a safe distance when passing them to avoid accidents.

If you have any further questions about this sign or other traffic rules, feel free to ask!