In this specific case, I believe that you should use:
an (obvious) problem
As MετάEd wrote in the question “A/An” preceding a parenthetical statement: "A parenthesis is a remark which you insert into the middle of a sentence as if you are interrupting yourself. A parenthesis contributes to the meaning of the sentence but interrupts and stands outside its syntax. In writing, we typically use curved brackets, dashes, or commas to mark a parenthesis."
Your example, a/an (obvious) problem, is not interrupted by the word obvious, i.e., this obvious functions as an adjective modifying problem, and thus the phrase is not parenthetical.
On the other hand, this is parenthetical:
Here is a (oh, you know I'd say that it's obvious!) problem.
Having said that, I believe that reworking the wording is another good option; for example, you can remove the parentheses entirely. You can also write the parenthetical part (which is more usual in formal writing) after the noun: a problem (an obvious one).
See also: Articles: “A” vs. “An” on Stack Exchange English Language & Usage
Whichever preposition is used here mean completely different things,
I travelled at the Philippines
Makes little sense and would not seem natural,
I travelled in the Philippines
Implies that you went to the Philippines that travelled around a bit.
The other alternative is talking about travelling to the Philippines which has a different denotation but may be what you're looking for:
I travelled to the Philippines
Implies that you went on a trip and the final destination was the Philippines.
In the end you have to choose what you mean by the sentence because each preposition changes its meaning.
Best Answer
1) roads as used here is a mass noun (roads in general cause some problem). So no article is used.
2) When you say:
it is referring to specific time based occasions. So you would use at. Same as:
This applies to the example you provided.
As for:
Here in is like during, so it would be referring to a period of time. For example: