No han llegado aún; habrá mucho tráfico en la autopista.

Han llegado is the present perfect: they have arrived. In Spain, this tense is very commonly used for past events that are still connected to the present moment.

So No han llegado aún means They haven't arrived yet, with a clear link to now.

If you said No llegaron, that would usually sound more like They didn't arrive, referring to a finished past event, not an ongoing situation.

Here aún means yet.

So:

• No han llegado aún = They haven't arrived yet
• No han llegado todavía = same meaning

In this sentence, aún and todavía are interchangeable.

A useful note:

• aún with an accent often means still/yet
• aun without an accent often means even

In everyday writing, though, you may sometimes see variation.

Spanish often omits subject pronouns because the verb ending already shows who the subject is.

• han llegado = they have arrived

So ellos is not necessary.

You could say Ellos no han llegado aún, but that would usually add emphasis or contrast, for example if you want to stress they specifically.

Grammatically, habrá is the future form of haber.

But in this sentence, it is very likely being used to express probability or guesswork, not a real future event.

So:

• Habrá mucho tráfico en la autopista literally looks like There will be a lot of traffic on the motorway
• but in context it often means There must be / there is probably a lot of traffic on the motorway

This is a very common use of the future tense in Spanish: using it to make a deduction about the present.

Because tráfico here is an uncountable noun, like traffic in English.

So Spanish treats it as singular:

• mucho tráfico = a lot of traffic

You would not normally say muchos tráficos when talking about cars on the road.

This is just a normal preposition difference between the two languages.

In Spanish, en is often used where English uses in or on for location:

• en la autopista = on the motorway
• en la calle = in/on the street
• en la playa = on the beach / at the beach

So even though English uses on, Spanish naturally uses en here.

Yes, both positions are possible.

You can say:

• No han llegado aún
• Aún no han llegado

Both are correct and mean the same thing.

Very roughly:

• Aún no han llegado can feel a little more focused on not yet
• No han llegado aún is also completely natural and common

The difference is usually small.

The semicolon links two closely connected ideas:

• No han llegado aún
• habrá mucho tráfico en la autopista

The second part explains or guesses the reason for the first part.

You could also see this written with:

• a full stop
• a comma in less careful writing
• sometimes even a colon

The semicolon simply shows a strong connection between the two clauses without fully separating them.

For a learner, autopista usually means motorway or highway.

In Spain, autopista and autovía are both major high-speed roads, but they are not always exactly the same in technical or legal terms. Very generally:

• autopista: motorway, often more controlled-access, sometimes toll road
• autovía: also a major divided highway, often public and free

In many everyday contexts, both can be translated simply as motorway.