It was my understanding that you didn't use indefinite articles (un/una) with professions in Spanish. "Soy maestro" not "Soy un maestro". But on Duolingo they always use articles. What's going on there?
Keith: Duolingo seems to be inconsistent on this. Sometime Duo uses the article, sometimes not. Of course, you are correct: The article should not be used with an unmodified profession. We need to report it each time to Duo under "Report a Problem". (BTW, the article IS used with a modified profession: Example: He is a teacher = Él es maestro. (no article)////////////but........... He is a good teacher = Él es un maestro muy profesional. (with article).
I've lived in the US all my life. I don't think anyone would seriously use either of those phrases. We call them traffic cops. Nothing more, nothing less. If you wanted to sound more official, you could call them "traffic officers", or explain that they are in the "traffic division", but "he is traffic police" simply does not make sense.
The term 'traffic warden' refers to someone who gives out tickets for parking and other minor enfringements. They are normally employed by the local council. They are not members of any police force and have very limited powers.
I don't think that you could consider the terms to be equivalent.
In the UK, we would probably use the term 'traffic cop', as mentioned by many others. More formally, we might say 'traffic officer', 'traffic policer officer' or 'member of the traffic police'.
Within the police service, they are apparently also referred to as 'road policing officers' or 'road officers'.
If you were talking about someone in (the Republic of) Ireland, this sentence could be translated as 'he's a traffic cop' or 'he's a member of the (Garda) traffic corps'.
I hope this helps.
In the US we are using words like “policeman” less than we did in the past because there is social pressure to use gender neutral terms. Thus “police officer” instead of “policeman” and “policewoman”; “worke’s compensation” instead of “workman’s compensation”; “hours of labor” instead of “man hours”; and so on. “Police” used alone implies the whole police department.