читается, просто вы еще не потратили много времени на кодирование

Depends what country you're in.

Переделывай - это хуйня не читаемая!

the interesting thing is that in real life yellow goes past red, not green :D

It's a complete misuse of ternary operators. You should only use ternary operators when you're going to return something with it.

it is advised not to use nested ternary operators because it might easily get confusing :) at least, that is what i learned

I did not even know you could nest ternary operators like this.

Is it really best for a traffic light to go 'green' as the default case?

Agreed. This solution only works because the cyclical state-change happens to only have 3 states. Rhetorically, how would it look if you had a traffic light with 10 colors?

Seems legit

In my opinion this is no best practice, because nesting a tenary operator in another one results in code that is very hard to read.