The law requirement will be a little different in different states.They should be marked with ANSI/ASAE S276.5, ANSI/ASAE S276.6 or ANSI/ASAE S276.7
You must display a slow-moving vehicle (SMV) sign as a warning to other drivers if your vehicle is not capable of reaching and sustaining a speed greater than 40 km/h.Vehicles that need a slow-moving vehicle sign include:
· construction equipment
· farm equipment
· street sweepers
· fork lifts
· Low-speed vehicles
The sign should be:
· placed on the rear of the vehicle between 0.6 meters (2 feet) and 2 meters (6 feet) above the roadway
· centred from left to right as much as possible
If your slow-moving vehicle is towing something, you need to display the sign at the rear of the object being towed.
Slow-moving vehicle signs should not be used on fixed objects such as mailboxes or driveway markers. You can be fined if you display a sign on a fixed object and the sign is visible from the road.