Learn Spanish (Latin America): Los torniquetes se cierran al mediodía.

Se cierran is an example of the reflexive passive (pasiva refleja). While son cerrados is grammatically correct, native speakers strongly prefer the reflexive passive for everyday actions where the person doing the action isn't important or isn't mentioned. It sounds much more natural than the direct English translation.

In phrases like se habla español, the verb is singular. Why is cierran plural here?

In the reflexive passive, the thing receiving the action (los torniquetes) is actually the grammatical subject of the sentence. Because los torniquetes is plural, the verb must be the plural cierran. If it were just one turnstile, it would be el torniquete se cierra.

Could this also translate to "The turnstiles close at noon" without the passive "are"?

Yes! With mechanical objects or doors, se cierran can be understood both as a passive action ("are closed by someone") and an action the object does itself ("close"). In Spanish, the reflexive covers both meanings effortlessly, so you don't need to distinguish between "are closed" and "close" in this context.

Why is it al mediodía rather than en el mediodía?