I'm a graduate student in transportation, working on mathematically modeling congestion and applying said model to calibrate how sensitive drivers are to differences in travel time between routes for microscopic traffic simulations. I'll be finishing up at the end of the semester, though. There's a lot of things I'd like to do, but if the question means to ask what I'd like to do for a living, it's difficult to answer, since I don't really have consensus. Part of me would like to continue in transportation applications, more specifically in vehicle to vehicle or vehicle to infrastructure integration, maybe driverless cars eventually. More realistically I'd like to do mathematical based stuff that improve people's lives, preferably in a position that allows me the free time to do what I want and in an area that I'd like to live in. While I have some idealist views with what I'd like to accomplish, a significant part of me wants to enjoy the activities and people that I enjoy - music, writing, reading, taking math tests for fun, watching movies, etc. I don't think I pursue any of those activities passionately enough to try to turn any of them into a career, but I definitely compromise my idealism with my personal enjoyment.
It sounds like you are doing interesting work, good luck with it. I'm sure this is old hat to you, but have you seen Bill Fords TED talk titled "Beyond Traffic Congestion" seems like it would be pertinent to your work. As for turning your passions in to your career, it's early yet... you never know what forks in the road will occur. Could be that you end up in music, writing or even taking math tests for a living. -Stranger things have happened.
I've seen it before, and just watched it again. It's unfortunate that transportation is politically a terrible issue, and also one that gets swept under the rug. By that I mean that most of the solutions wanted by the transportation minds are politically infeasible (gas tax, restrictions on parking), yet it remains a pressing issue that never gets talked about by anybody, except a few high speed rail proposals every now and then (that I'm not an expert on). I actually spoke to a guy from Ford working on some future mobility stuff at a transportation career fair, but already had something worked out for the next year or so. We'll see what happens - a lot of the cooler research based stuff probably requires a PhD and more passion to accomplish, and I'm definitely not prepared for that. It's a weird transition from not having a clue what I wanted to do (back in undergrad) to the slow realization that I have access to more than I thought previously (now in grad school), to transportation or math-based careers. It's a great problem to have, but making important life decisions stresses me out. Maybe they aren't as important as I think - I'm in my early 20's, so there's plenty of time to figure things out.
I wouldn't say that it's not an important decision, I'd say it's quite important. That said it's not a binding decision by any means. People change their careers often, and it would be shocking if you were still doing the same thing when you're 50 that you did when you were 25. Good luck with all of your decisions, I hope you do something that's intellectually satisfying and is able to make a positive difference. You're in a good field for that.