Leadership case study

The Situation:

You are in a patrol truck checking on pavement conditions after an ice storm. You see John passing by in a heavy dump truck with the plow and wing up. H is just spreading salt on this part of his shift   In you opinion there is enough accumulation to warrant plowing. A short time late, at the shop, you take him aside to discuss this with him. You tell him that under these conditions he should always be clearing snow with the plow and wing. John becomes upset and tells you that he is certified to do his job and he knows how to handle his shift. He says he has been plowing snow since you were in diapers. He asked you if you are giving him an order or just giving him a hard time.

Question:

What do you say to John? 

What should have happened differently