slow-moving or stationary vehicle (such as a Traffic Officer vehicle, salt spreader, snow plough or recovery vehicle) or abnormal loads, so approach with caution. On unrestricted dual carriageways, motor vehicles first used on or after 1 January 1947 with a maximum speed of 25 mph (40 km/h) or less (such as tractors) MUST  use a flashing amber beacon (also see Rule 220 above). 

Law RVLR 1989, reg 17 

