Horse-Drawn Vehicles Ahead Post Mounted Sign - 550.2 R2/RA2 (Face Only)Where horse drawn vehicles such as a buggy or carriages are likely to use the road ahead, diagram 550. 2 is the appropriate warning sign. This is to warn drivers to watch their speed as horses may get frightened from loud noises or fast moving vehicles. If you're looking for a sign to warn of wild horses 550 should be used, or ridden horses 550. 1. This triangular warning sign is less common, only being used where horse powered transport is common