Dans ce virage, il vaut mieux ne pas accélérer, parce qu’on ne peut pas doubler facilement.

Dans ce virage means in this bend/curve/turn.

French uses dans because it often expresses being in a place or situation. With roads, curves, bends, intersections, etc., French commonly uses dans where English might sometimes prefer on, at, or simply no preposition depending on the wording.

• dans ce virage = in this bend
• dans cette rue = in this street
• dans un tunnel = in a tunnel

Here, ce virage is masculine singular, so the demonstrative adjective is ce.

French uses:

• ce before a masculine singular noun beginning with a consonant
• cet before a masculine singular noun beginning with a vowel or mute h

Since virage begins with v, a consonant, it is:

• ce virage

Compare:

• cet accident
• cet hôtel

Il vaut mieux means it is better or it’s best.

It comes from the verb valoir (to be worth), but in this expression you should think of it as a fixed phrase:

• Il vaut mieux partir maintenant. = It’s better to leave now.
• Il vaut mieux ne pas attendre. = It’s better not to wait.

This structure is very common in French for giving advice in a softer, more general way than a direct command.

The il here is an impersonal subject. It does not refer to a person or thing.

French often needs a subject even in general expressions, so it uses il in phrases like:

• Il faut... = It is necessary / one must...
• Il pleut. = It is raining.
• Il vaut mieux... = It is better...

So this il is just part of the grammar of the expression.

Because after il vaut mieux, French normally uses the infinitive when the subject is general or unspecified.

So:

• Il vaut mieux ne pas accélérer. = It’s better not to accelerate.

This is similar to English It’s better not to accelerate, where to accelerate is also not a finite verb.

If you wanted to specify a person, French could use another structure, but here the general advice is naturally given with the infinitive.

When French negates an infinitive, ne and pas are placed directly before it:

• ne pas accélérer = not to accelerate
• ne pas fumer = not to smoke
• ne jamais oublier = never to forget

So in this sentence, ne pas accélérer is one infinitive phrase meaning not to accelerate.

Because que becomes qu’ before a vowel sound.

Since on begins with a vowel, French contracts:

• parce que on → parce qu’on

This is very common:

• je pense qu’il vient
• parce qu’elle est là
• qu’on ne peut pas

It is mainly about pronunciation and normal written form.

Here on means something like you, we, or people in general, depending on context.

In this sentence, it is best understood as a general statement:

• on ne peut pas doubler facilement = you can’t overtake easily / it’s not easy to overtake

French often uses on where English uses a general you or a passive/general idea.

It can also mean we in everyday French, but here it is more general: anyone driving in that bend would have the same problem.

Because the subject is on, which takes third-person singular verb forms.

So:

• je peux
• tu peux
• il/elle/on peut
• nous pouvons

Since the sentence says on ne peut pas, the correct form is peut.

In driving vocabulary, doubler usually means to overtake or to pass another vehicle.

So here it does not mean to make something twice as much. It means:

• doubler une voiture = to overtake a car
• interdiction de doubler = no overtaking / no passing

This is a very common road-related use of doubler.

Facilement is an adverb meaning easily. In French, adverbs often come after the verb or after the infinitive phrase they modify.

So:

• doubler facilement = to overtake easily

This word order is very natural in French. English also often does the same here: to overtake easily.

Yes. Il ne faut pas accélérer is possible, but it is stronger.

• Il vaut mieux ne pas accélérer = It’s better not to accelerate.
  This sounds like advice or a recommendation.
• Il ne faut pas accélérer = You must not / should not accelerate.
  This sounds more like a rule, warning, or prohibition.

So the sentence you have is a bit softer and more explanatory.

The comma separates the main idea from the reason that follows:

• Dans ce virage, il vaut mieux ne pas accélérer,
• parce qu’on ne peut pas doubler facilement.

It helps readability and reflects a pause in speech. French punctuation here is fairly similar to English punctuation in a sentence like:

• In this bend, it’s better not to accelerate, because you can’t overtake easily.

Not every similar sentence must have a comma, but here it is perfectly natural.