Se a ultrapassagem for perigosa, é melhor travar cedo.

Because after se meaning if, Portuguese often uses the future subjunctive when talking about a possible future situation.

So:

• Se a ultrapassagem for perigosa... = If the overtaking manoeuvre is / turns out to be dangerous...

Here, for is the future subjunctive form of ser.

Compare:

• Se é perigosa = if it is dangerous
  
  • This sounds more like a general fact or a statement about something already known.
• Se for perigosa = if it is dangerous / if it turns out to be dangerous
  
  • This is the normal choice for a possible future case.

In this sentence, the speaker is talking about what to do in a situation that may happen, so for is the natural form.

For is the future subjunctive of the verb ser.

The future subjunctive is very common in Portuguese after words like:

• se = if
• quando = when
• logo que = as soon as
• assim que = as soon as

Examples:

• Se for necessário, telefono. = If necessary, I’ll call.
• Quando ele chegar, começamos. = When he arrives, we’ll start.

For ser, some useful forms are:

• eu for
• tu fores
• ele/ela/você for
• nós formos
• vós fordes (rare in modern use)
• eles/elas/vocês forem

So in your sentence, a ultrapassagem is singular, so it takes for.

Ultrapassagem means overtaking or passing in a traffic/driving context.

It refers to the act of going past another vehicle.

In Portugal, this is the standard word you will see in driving language, road signs, and theory tests.

Examples:

• É proibida a ultrapassagem. = Overtaking is forbidden.
• Ultrapassagem perigosa = dangerous overtaking

So here, a ultrapassagem means the overtaking manoeuvre or the passing manoeuvre.

Portuguese uses definite articles more often than English.

So a ultrapassagem literally means the overtaking, but in English we might translate it more naturally as:

• if overtaking is dangerous
• if the overtaking manoeuvre is dangerous
• if the pass is dangerous

Portuguese often prefers the article with nouns used in a general or specific-situation sense. In a driving instruction sentence like this, a ultrapassagem sounds completely natural.

Because perigosa agrees with ultrapassagem, which is a feminine singular noun.

• a ultrapassagem = feminine singular
• perigosa = feminine singular adjective

Portuguese adjectives usually agree with the noun in gender and number.

Compare:

• a manobra perigosa = the dangerous manoeuvre
• o cruzamento perigoso = the dangerous junction
• as ultrapassagens perigosas = the dangerous overtakes

So the -a ending is there because the noun is feminine.

É melhor + infinitive is a very common way to say it is better to...

So:

• é melhor travar cedo = it is better to brake early

This structure is impersonal, just like English it is better to...

Other examples:

• É melhor esperar. = It is better to wait.
• É melhor sair agora. = It is better to leave now.
• É melhor não falar. = It is better not to speak.

You do not need to mention a subject like you. Portuguese often leaves that general meaning implicit.

Here travar means to brake.

In European Portuguese, travar is a common verb in driving contexts. It can mean:

• to brake
• to stop by braking
• sometimes more generally to lock / jam, depending on context

In traffic language:

• travar cedo = to brake early
• travar a fundo = to brake hard

You may also come across frear, especially in Brazilian Portuguese, but in Portugal travar is very natural.

Here cedo means early.

So:

• travar cedo = to brake early

It does not mean early in the day here. It means soon enough / in good time / before it is too late.

A few examples:

• Chega cedo. = Arrive early.
• Acordei cedo. = I woke up early.
• Travou cedo para evitar o acidente. = He braked early to avoid the accident.

Because the sentence is giving general advice, not speaking directly to one named person.

Portuguese often uses impersonal expressions like:

• é melhor... = it is better...
• convém... = it is advisable...
• é preciso... = it is necessary...

So instead of saying:

• you should brake early

Portuguese naturally says:

• é melhor travar cedo

The idea is still understood as advice to the driver, but the sentence stays general and neutral.

Grammatically, yes, but it usually means something slightly different.

• Se a ultrapassagem for perigosa...
  = If the overtaking manoeuvre turns out to be dangerous / if it is dangerous in that situation

  • This is the normal choice for a possible future condition.

• Se a ultrapassagem é perigosa...
  = If overtaking is dangerous

  • This sounds more like a general statement or a less idiomatic way of expressing the idea in this context.

In driving instructions, se... for... is the more natural phrasing.

Yes, the comma is normal here because the sentence begins with a conditional clause:

• Se a ultrapassagem for perigosa, = If the overtaking manoeuvre is dangerous,
• é melhor travar cedo. = it is better to brake early.

In Portuguese, when a se clause comes first, it is usually separated by a comma.

Compare:

• Se chover, ficamos em casa. = If it rains, we stay at home.
• Se vires o João, avisa-me. = If you see João, let me know.

If the main clause comes first, the comma is often omitted:

• É melhor travar cedo se a ultrapassagem for perigosa.

A little, but the given order is the most natural.

Standard order:

• Se a ultrapassagem for perigosa, é melhor travar cedo.

You could also say:

• É melhor travar cedo se a ultrapassagem for perigosa.

That is also correct.

But moving cedo around too much can sound awkward:

• É melhor cedo travar → possible in theory, but not natural here

So the original sentence is a very normal and idiomatic word order for Portuguese.

If you wanted a more direct command, Portuguese might say something like:

• Se a ultrapassagem for perigosa, trave cedo.
  = If the overtaking manoeuvre is dangerous, brake early.

That uses the formal imperative/você form trave.

But é melhor travar cedo sounds softer and more like advice or guidance, which is very common in instructional material.