I think you should first determine some area to help people focus on specific subject. If you ask people what are your problems ? Most of them won't be able to answer you.

If you propose some context, it easier for people to provide feedback. For example, you could ask, is there something that bothers you all the time in the metro? If yes, tell us more about it. If no, is there another place where something particular is bothering you?

One way of determining those situation and target groups of people would be to crawl forums/groups where there are tons of people complaining about their problems. List those issues and try to get more feedback from those people ! This is how Airbnb got started by the way...