A placa dizia que havia pedágio na frente e também avisava sobre engarrafamento.

Because dizia and avisava are in the imperfect past. In this sentence, that gives the idea of what the sign said / was saying and was warning about as a general statement, not as one single completed action.

With signs, notices, and written text, Portuguese often uses the imperfect this way:

• A placa dizia... = The sign said...
• O bilhete dizia... = The note said...

Using disse or avisou would sound more like a specific completed event:

• A placa disse... is grammatically possible, but much less natural here.

So dizia and avisava fit the idea of ongoing written information on a sign.

Here, havia means there was / there were.

It comes from the verb haver, which is often used in Portuguese to express existence:

• Havia pedágio = There was a toll
• Há um problema = There is a problem

This is a very common structure in formal or neutral Portuguese.

In everyday Brazilian Portuguese, many speakers would also say:

• tinha pedágio na frente

That is very common in speech, even though havia is more formal or standard in writing.

Both are possible, but they are slightly different in feel.

• havia pedágio na frente = there was a toll / toll collection ahead, focusing on the existence of tolling as information
• havia um pedágio na frente = there was a toll booth / one toll point ahead, sounding a bit more specific and countable

Without the article, Portuguese often sounds more general or informational, which works well on road signs and notices.

Literally, na frente can mean in front or at the front, but in this road context it means ahead.

So:

• pedágio na frente = toll ahead
• engarrafamento na frente = traffic jam ahead

It is a very natural expression when talking about what is further along the road.

Placa here means a sign or road sign.

It can refer to different kinds of signs or plates depending on context:

• placa de trânsito = traffic sign
• placa do carro = license plate

In this sentence, because of pedágio and engarrafamento, it clearly means a road sign.

Here, que means that and introduces a subordinate clause.

• A placa dizia que havia pedágio...
• literally: The sign said that there was a toll...

Just like in English, that can sometimes be omitted in translation:

• The sign said there was a toll ahead

But in Portuguese, que is normally kept in this kind of sentence.

Também means also or too.

It tells you that the sign gave another piece of information in addition to the toll:

• it said there was a toll ahead
• it also warned about a traffic jam

So the sentence has two coordinated ideas:

1. dizia que havia pedágio na frente
2. também avisava sobre engarrafamento

Because avisar sobre means to warn about / inform about something.

• avisar sobre engarrafamento = warn about a traffic jam
• avisar sobre chuva = warn about rain

If you say avisar alguém de algo or avisar algo a alguém, the structure changes:

• Avisei meu amigo do engarrafamento. = I warned my friend about the traffic jam.
• A placa avisava os motoristas sobre o engarrafamento. = The sign warned drivers about the traffic jam.

In your sentence, sobre is used because the thing being warned about is the topic.

Both can work, but the meaning shifts slightly.

• sobre engarrafamento sounds more general: warning about traffic congestion
• sobre o engarrafamento sounds more specific: warning about the traffic jam

Road-sign style language often drops the article to sound more general, concise, or informational.

So sobre engarrafamento feels a bit like sign language or headline style, while sobre o engarrafamento would point more clearly to a specific traffic jam.

Engarrafamento means traffic jam.

It comes from engarrafar, which literally has the idea of bottling up or getting stuck like in a bottle, which helps explain the image of cars packed tightly together.

Very common related words are:

• trânsito = traffic
• congestionamento = congestion
• engarrafamento = traffic jam

In everyday Brazilian Portuguese, engarrafamento is extremely common.

In standard Portuguese, when haver means to exist, it is used only in the singular.

So:

• havia pedágio
• havia muitos carros
• havia problemas na estrada

Not:

• haviam muitos carros in standard grammar

This is important because haver in this use has no real subject. It works like there is / there are in English.

Yes. A more conversational Brazilian Portuguese version might be:

• A placa dizia que tinha pedágio na frente e também avisava sobre engarrafamento.

Or even:

• A placa avisava que tinha pedágio mais à frente e engarrafamento também.

The original sentence is perfectly natural, but havia sounds a bit more formal or written than tinha, especially in Brazil.

All three are possible, but they have slightly different tones:

• na frente = ahead / in front
• à frente = ahead, often a bit more formal or careful
• mais à frente = further ahead

Examples:

• Havia pedágio na frente.
• Havia pedágio à frente.
• Havia pedágio mais à frente.

On roads and signs, à frente and mais à frente are especially common. Na frente is also understandable and natural, though sometimes it can sound a bit more literal depending on context.