You have a starting point now its time to extend it past that. What about the idea that interests you? How can you explore it?
Pursue the concept through a range of medias, just because you do photomanipulation doesn't mean you can't do painting. A Body of Work allows for a wide range of exploration.
The best thing you can do is start to collect ideas of pursuing the concept. Using a range of media areas help as do taking a different spin on things.
"The Tragedies of Young Drivers" gives you many starting points. You can focus on the tragedies, but don't make it too obvious and in the face of the audience, subtlety can be powerful.
You don't have to make a full blown social commentary. At this stage even small things such studies of vehicles, the framework of wheels and cars can work. Maybe even showing the distortion of the perfect 'wheel' parralleling the result of a car crash.
Best thing to do is approach your idea in a way that people can't scream cliche. Take the unexpected route. You can focus on the sadness and the waste of life, etc... but that can sometimes be a bit too common. What I mean is social issues like suicide, etc is tackled alot so best to surprise the examiner. Weave a little amibuity into your work so it can be read from mulitple viewpoints, this will make your work stronger.
I like to make a mindmap of all the different paths a concept can go. Once you have about five solid directions you can work on them. Don't worry ideas will develope over time. But it is best to have a general idea sorted out before Term 4's end.
So I say keep on thinking of ideas, but also continue your art making. If you have a idea run with it and see how it is in a week's time.
Also maybe explore the nature of "Tradegy" and how it is present (now and in the past). Narrowing your focus can prevent your concept from developing so becareful.
Maybe focus on the nature of life and how vehicles can take it away from us. Markers like these little quirks.
Sorry I can't proof read this right now, I know I've repeated myself but its 1:00 am and I'm tired... Xp