'Highway' may be one of the most ambigious terms over here. Now, North Americans may suggest that it is not, but my impression is that they really do not care about precission. Indeed, that may be one of English's greatest flaws: It is not very precise. And not a very good technical language. But alas, here we are, using English. (Everything would be better if everyone spoke German.)
The thing about 'highway' is that it basically refers to any larger country road. Be it a secondary or primary route, or a dual carriageway or even a motorway. In my mind, those are four different types of roads, but over here; they are the same. Or rather, referred to by the same term: Highway.
Which makes the term useless. But let us not rid ourselves of it so quickly. Indeed. I propose to use 'highway' to refer only to secondary and primary routes. That is, significant country roads. Or - to be more precise, as this language would we would rather not - country roads with a route number. Be it a federal, a state or a county road, they are all highways. Now that is easy to comprehend.
Dual carriageways are called 'divided highways'. I cannot decide on which I prefer, so I default to the British; dual carriageway. Wikipedia struggles to compromise on motorway, so its article on Wikipedia is called 'access controlled highway'. That does not flow off the tongue. Secondly, while a highway can refer to a motorway over here, they might also call them 'freeway', 'thruway', 'expressway' or 'turnpikes'. But they are all access controlled highways, and therefore motorways. If you pay toll, we call it a toll motorway.
While I am on the subject of fixing the English language, motorway exits. In the UK, these are called junctions. Weird. In the US, they are called exits. Makes more sense. But in the US, 'exits' also refers to forks in the motorway, where you switch to another motorway route number. But that is not an exit; I will not be leaving the road type after all. Exits are only for when you leave the road type!
So let us clear this up: Junctions refer to when motorways meet one another, or - for precision's sake (Shakespeare must be spinning in his grave) - motorway junction. Forks refer to when the motorway divides into two or more motorway routes; usually at a motorway junction. And exits refer to when you leave the motorway, that is onto a differen type of road. Such as highway, perhaps.
There is no need to thank me.
If you have read this far, you may be inclined to think why road types and terms matter. Again, precision. But a proper understanding of different road types, also helps one to understand the different behaviours one should strive for on them. And let me just clarify, I have seen a lot of bad driving behaviour over here.
Note: Hopefully I will add pictures later. But I am really arriving late at hotels these days and getting out early, so I do not really have much time to write these!