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I came across the following exercise – I have to fill in the gap with the
correct word that I have to choose from a series of 4.

> 信号{しんごう}があおに（ ）、道{みち}を渡{わた}っていいです。

I eliminated 2 words and now I'm stuck with: なったら and なると. In my mind both #1
and #2 make sense.

> 1.信号があおになったら、道を渡っていいです。

_If the traffic light becomes / will become blue, it is ok to cross the road._

> 2.信号があおになると、道を渡っていいです。

_When the traffic light will become blue, it is ok to cross the road._

Which one is correct and why?

P.S. The book says なったら is the correct one.
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question
grammar word-choice usage
なったら vs. なると – usage in context
You can only use なったら, not なると.

First, take a look at [this
topic](https://japanese.stackexchange.com/questions/393/differences-
among-%E3%81%9F%E3%82%89-%E3%81%AA%E3%82%89-%E3%82%93%E3%81%A0%E3%81%A3%E3%81%9F%E3%82%89-%E3%81%88%E3%81%B0-etc),
and you see how they exactly describe the difference of と and たら.

> と, ば: The main clause must be a **constant non-volitional reaction** to the
> conditional clause unless the conditional clause shows state or if the
> subjects of the two clauses differ.
>
> ～たら 1. Use when expressing a **one-off (as opposed to constant or general)
> dependency**.
>
> (emphasis added)

Now, why can't you use と in your case? Because the ～て(も)いい is, basically, an
expression giving a permission _ad hoc, each specific time you say the word_.
Thus your example #2 would sound like:

> ?? _You may cross the road now whenever the light turns green._

If you'd like to paraphrase it correctly with と, you'd say:

> 信号があおになると、道を(渡れます/渡ることができます)。 _" You can cross the road when ..."_  
>  信号があおになると、道を渡っていいことになります。 _" When ..., it means you may cross the road
> then."_

It nevertheless **doesn't** mean you can't make a general description with
～ていい. It's free to express a general idea taking the form of commenting about
a particular instance, but as long as you do so, you have to pretend it
through. It's like we could say `Does a wombat bite?` instead of `Do wombats
bite?`, but never `*Do a wombat bite?` (nor `*Does wombats bite?`).

_**Acknowledgement**_ : Thanks to @DariusJahandarie for his help and advice
improving this answer.
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answer
"なったら" is correct, because "なったら" describes conditions, on the other hand
"なると" describes subsequent events.

```

    信号{しんごう}があおに（　　　　）、道{みち}を渡{わた}っていいです。
    
```

"いいです" means "allow" or "feel free to do", so the former half of this sentence
imposes a condition to be able to cross the road. You can make a choice freely
whether you cross the road or not.

"なると" means "When A occurs, B almost always occurs". The latter half of the
sentence you give says about something you can do, not about some events which
almost always occurs after the traffic light becomes blue. So "なると" is
incorrect.
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answer