You can only use なったら, not なると.
First, take a look at this topic, 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.
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.