*To be perfectly honest, you were being a complete pain in the arse. If I behaved like that my DH would be pissed off and would probably have left me before now.
He shouldn't have hit you but he was badly provoked. You also say "we hadn't even argued" as though this is a common occurrence.
You need to find out why he has been so miserable lately. Perhaps he's deeply unhappy.*
Are you the OP's husband?
If someone is annoying you while you are driving, you pull over, you say 'don't do that while I'm driving, it's distracting and dangerous' then you carry on with your day.
Nothing justifies punching someone in the face, it's an act of extreme violence and done in front of the children makes it worse. He may be stressed, he may be unhappy - lots of people are, they don't punch their loved ones.
Anyone who makes any kind of excuse for this appalling behaviour is victim blaming and condoning domestic violence.
Please don't accept any excuses from him OP, he WILL do it again - it's always easier for them after the first time.
Contact women's aid, or contact the police - they will give you advice on keeping yourself safe and put you in touch with organisations who can help you.
You don't need to live with this, it is not normal, it is not acceptable - don't let yourself get trapped in an abusive and violent relationship.
This is your warning call to get out before this becomes your life, you deserve better than this and so do your children.