Some vehicles by their nature are slow and seeing them is a hazard clue (mopeds, farm machinery, construction machinery, tractors, etc.). A red triangle with an orange center is a hazard clue or symbol for "slow moving vehicle" (SMV).
SMV emblem via kwout
A vehicle displaying this sign may not be operated faster than 25 mph.
It’s legal to drive farm machinery on public roads and it’s often the only way farmers can get from field to field. So if you’re heading out to the rural areas to pick pumpkins and apples or check out the fall colors, you’re likely to encounter slow moving farm equipment on the road.
Make sure to be aware of farm equipment and slow down when encountering slow moving farm equipment. See the things you must remember when driving on rural roads at Uwex.edu.
Check out Safety & Driving Tips from DMV.org.