Slippery Road Ahead Post Mounted Sign - 557 R2/RA2 (Face Only)The skid risk warning sign to diagram 557 alerts drivers where the road is slippery and the danger of a vehicle skidding is greater than usual. This is usually where drainage is poor or the surface is covered in chippings or is of poor quality. A plate For and a distance may be supplemented. Detailed guidance on road conditions can be found in HD 28, Skidding Resistance, in Volume 7 of DMRB if you are unsure if a special warning is necessary.