Maak die veiligheidsgordels vas; steek nie in swaar verkeer verby nie.

Because the dictionary form is vasmaak (to fasten), and this is a separable verb.

In a main clause or command, Afrikaans often splits this kind of verb:

• vasmaak → Maak ... vas
• toemaak → Maak ... toe
• oopmaak → Maak ... oop

So Maak die veiligheidsgordels vas literally behaves like Fasten the seat belts up/secure.

Afrikaans often uses the article (die) in instructions where English would use your or no article at all.

So:

• Maak die veiligheidsgordels vas = Fasten your seat belts / Fasten seat belts

This is very normal in commands and notices. The context already makes it clear whose seat belts are meant.

It is plural because this is a general safety instruction aimed at everyone in the vehicle, not just one person.

• veiligheidsgordel = seat belt (singular)
• veiligheidsgordels = seat belts (plural)

A sign or announcement often uses the plural because multiple passengers are being addressed at once.

Because this is an imperative (a command), and Afrikaans usually leaves the subject out in commands, just like English often does.

Compare:

• English: Fasten seat belts.
• Afrikaans: Maak die veiligheidsgordels vas.

The subject is understood:

• jy = you (singular, informal)
• julle = you (plural)
• sometimes u in more formal situations

That is the normal Afrikaans double negative pattern.

Very often, Afrikaans negation is built with:

• one nie after the verb or verbal part
• another nie at the end of the clause

So:

• steek nie ... verby nie = do not overtake ...

This is one of the most important grammar patterns in Afrikaans.

Yes — that is the form most learners are taught first, and it is the safest general-purpose form to use.

A fuller, very standard version would be:

• Moenie in swaar verkeer verbysteek nie.

The sentence you were given is a shorter, more compressed instruction style. So a learner should definitely recognize moenie ... nie as the usual negative-command pattern, even if shorter warning-style wording also appears.

On its own, steek can have several meanings. In this sentence, it is part of the expression verby steek / verbysteek, which means to overtake or to pass a vehicle.

So the meaning comes from the whole expression, not from steek alone.

This is common in Afrikaans: a verb plus a particle can create a more specific meaning.

Because verbysteek is another separable verb.

Its full form is:

• verbysteek = to overtake

But in a main clause, it splits:

• steek ... verby

So:

• Hy steek die bus verby. = He overtakes the bus.
• Steek nie in swaar verkeer verby nie. = Do not overtake in heavy traffic.

This is the same kind of pattern as vasmaak → maak ... vas.

Literally, it means in heavy traffic.

• swaar = heavy
• verkeer = traffic

Afrikaans uses swaar in much the same way English uses heavy in expressions like heavy traffic.

It separates two closely related commands:

• Maak die veiligheidsgordels vas
• steek nie in swaar verkeer verby nie

The semicolon shows that these are two distinct instructions, but they belong together as part of the same safety message.