Kad vidiš zastoj na obilaznici, uspori.

Here kad means when.

Kad and kada are basically the same in meaning. The difference is mostly style:

• kad is shorter and very common in everyday speech
• kada sounds a bit more formal or a bit more explicit

So Kad vidiš zastoj na obilaznici, uspori. and Kada vidiš zastoj na obilaznici, uspori. both work.

That is very normal in Croatian.

After words like kad (when), Croatian often uses the present tense to refer to something that will happen in the future:

• Kad dođe, nazovi me. = When he comes, call me.
• Kad vidiš zastoj, uspori. = When you see a traffic jam, slow down.

So vidiš is grammatically present, but in this kind of sentence it refers to a future event.

Vidiš is 2nd person singular of vidjeti.

That means it corresponds to you see.

The ending -iš tells you the subject is you singular:

• vidim = I see
• vidiš = you see
• vidi = he/she/it sees

Because Croatian usually does not need subject pronouns.

The verb ending already tells you who the subject is:

• vidiš = you see
• uspori = slow down (addressed to one person)

You could add ti for emphasis, but normally it is omitted:

• Kad ti vidiš zastoj... would sound marked or contrastive, not neutral.

Zastoj is the direct object of vidiš, so it is in the accusative singular.

However, zastoj is a masculine inanimate noun, and for many masculine inanimate nouns in Croatian, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: zastoj
• accusative: zastoj

That is why the form does not change.

Because Croatian does not have articles like English a/an and the.

So zastoj can mean:

• a traffic jam
• the traffic jam

The exact meaning is understood from context.

That is one of the biggest differences from English.

Because na can take different cases depending on meaning.

Here it means location: on the bypass / on the ring road, so Croatian uses the locative:

• na obilaznici = on the bypass

If you were talking about motion onto the bypass, you would use the accusative:

• na obilaznicu = onto the bypass

So:

• zastoj na obilaznici = a traffic jam on the bypass
• idem na obilaznicu = I’m going onto the bypass

Obilaznici is the locative singular of the noun obilaznica.

The base form is:

• obilaznica = bypass, ring road

Since it is a feminine noun ending in -a, the locative singular here is:

• obilaznica → obilaznici

Uspori is the imperative, 2nd person singular.

It means:

• slow down

It is a command or instruction addressed to one person.

Related forms:

• uspori = slow down (to one person)
• usporite = slow down (to several people, or polite/formal singular)

This is about aspect, which is very important in Croatian.

• uspori is a perfective imperative: it tells someone to perform the action, to reduce speed
• usporavaj is imperfective: it would sound more like be slowing down, slow down repeatedly/continuously

In a short warning or driving instruction, uspori is the natural choice because the speaker wants one clear action: reduce your speed now.

Because the sentence begins with a subordinate clause:

• Kad vidiš zastoj na obilaznici = subordinate when clause
• uspori = main clause

Croatian normally separates that kind of introductory clause with a comma.

So the structure is:

• When you see a traffic jam on the bypass, slow down.

Yes. Croatian word order is fairly flexible.

The given version is neutral and natural:

• Kad vidiš zastoj na obilaznici, uspori.

But other orders are also possible, for example:

• Kad na obilaznici vidiš zastoj, uspori.
• Uspori kad vidiš zastoj na obilaznici.

These versions are all understandable, but the original sentence sounds like a normal, clear instruction.

Mostly when, but in practical instructions it can feel a little like whenever or even if.

In a sentence like this, the idea is:

• if/when this situation happens, do this

Croatian often uses kad very naturally in that kind of instruction:

• Kad vidiš zastoj, uspori.

If you used ako, the sentence would sound more explicitly conditional:

• Ako vidiš zastoj na obilaznici, uspori. = If you see a traffic jam on the bypass, slow down.

Both are possible, but kad sounds very natural in warnings and instructions where the situation is treated as something expected.