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.