Agricultural Vehicles Post Mount Sign- 553.1 R2/RA2 (Face Only)The tractor road sign diagram 553. 1 warns drivers that agricultural vehicles, such as tractors, regularly cross or use the road at low speed. This sign should be used wherever a farm vehicle is likely to cause a hazard, and should especially be considered where it is hard to see because of a blind bend, or where these vehicles cross a high speed road. Where the vehicle regularly travels between two entrances, a distance plate to show how far the