Antes de girar, hay que ceder el paso a los peatones.

Because antes de + infinitive is a very common Spanish structure meaning before doing something.

So:

• antes de girar = before turning

In Spanish, when the subject is general or the same as in the main idea, the infinitive is natural here.

Compare:

• Antes de salir, apaga la luz. = Before leaving, turn off the light.
• Antes de comer, lávate las manos. = Before eating, wash your hands.

If Spanish needs to mention a different subject, it often uses antes de que + subjunctive instead:

• Antes de que gires, mira por el espejo. = Before you turn, look in the mirror.

Hay que is an impersonal expression meaning:

• one must
• you have to
• it is necessary to

So:

• hay que ceder el paso = you have to yield / one must give way

It does not mean that a specific person has something. It is not the same as tener in the ordinary sense.

For example:

• Tengo que estudiar. = I have to study.
• Hay que estudiar. = You have to study / One must study.

In your sentence, hay que sounds like a general rule, which fits traffic instructions very well.

Because hay que is a fixed impersonal structure. It always uses hay, from haber, in the third-person singular form.

So even if the rule applies to everyone, Spanish still says:

• Hay que esperar.
• Hay que parar.
• Hay que ceder el paso.

You do not change it to match who is affected. It works like an impersonal you must or one must in English.

Ceder el paso is a fixed traffic expression meaning:

• to yield
• to give way
• to let someone go first

Literally, it is something like to yield the passage/right of way.

In road language, this is the normal phrase used in Spain for yielding to another road user.

Examples:

• Hay que ceder el paso en esta intersección. = You must yield at this junction.
• Cedió el paso al peatón. = He/She yielded to the pedestrian.

So in your sentence, ceder el paso a los peatones means to yield to pedestrians.

Because the usual expression is ceder el paso, with the article included. It is the standard set phrase.

Spanish often uses articles in expressions where English does not. Here, el paso is part of the idiom.

So the natural form is:

• ceder el paso a alguien

not normally:

• ceder paso a alguien

In road signs and formal language, ceder el paso is the standard wording.

That a is there because the pedestrians are the indirect target/recipient of the action in the expression ceder el paso a alguien.

You yield the right of way to someone, so Spanish uses a:

• ceder el paso a los peatones
• ceder el paso al coche
• ceder el paso a la ambulancia

This is also consistent with the so-called personal a when people are involved, but in this phrase the a is especially expected because of the structure ceder algo a alguien.

Spanish often uses the definite article to talk about a whole class of people in a general sense.

So:

• los peatones = pedestrians in general

This is very normal in Spanish. English often drops the article where Spanish keeps it.

Other examples:

• Los niños necesitan dormir bien. = Children need to sleep well.
• Los conductores deben tener cuidado. = Drivers must be careful.

So a los peatones here means to pedestrians, understood as pedestrians in general.

Yes. Girar is a very normal and standard word in driving and road contexts in Spain.

Examples:

• girar a la derecha = turn right
• girar a la izquierda = turn left

Depending on the region, you may also hear other verbs such as doblar in some places, but girar is very standard and especially common in formal driving language, instructions, and traffic rules.

Grammatically, antes de que + subjunctive is possible, but your example sounds a bit mixed because gires addresses you, while hay que is impersonal.

A more natural pairing would be one of these:

• Antes de girar, hay que ceder el paso a los peatones.
  = general rule

• Antes de que gires, tienes que ceder el paso a los peatones.
  = speaking directly to one person

So yes, antes de que gires is possible, but it fits better when the rest of the sentence also refers to a specific person.

Because Antes de girar is an introductory phrase at the beginning of the sentence, and Spanish often separates that kind of opening element with a comma.

So:

• Antes de girar, hay que ceder el paso a los peatones.

This makes the sentence easier to read. In short sentences, punctuation can sometimes vary, but the comma here is completely natural and standard.