Flawed, but not bad
I really like the idea of this and you've got most of the basics sorted, which is great. However, there are a few significant problems I've found with the game - hopefully you can improve on the following:
Selecting your character doesn't always work properly. For example, I chose the guy with the tiny red eyes and the massive black hair/helmet, but in the game it showed me as the weird red lips thing.
There is a very-easily exploitable bug in the game's code - you can simply start a course and reverse/accelerate over the blue starting line a few times to finish the race. While I don't know anything about making this kinda game in Flash, I'm assuming that when the player's vehicle makes contact with the bar the game registers this as a completed lap. Seeing as how you can simply cheat and complete a 3-lap course in about five seconds, you probably want to fix this!
Another problem with the lap system is that the course will end when you start the third lap, not when you've finished it. This again appears to be a problem caused by the starting bar - because you get placed right before it at the start of a course, you have to go through it, and this makes the game think you've completed your first lap. I'm sure there's a more solid way of fixing this, but in the mean time you could simply make the player appear in front of the bar.
Other than that, it's just minor stuff like perhaps having a few different music tracks and adding stats for the different vehicles. Still, what you've got here is pretty good and despite the obvious flaws it's actually pretty fun to play.
I really look forward to seeing the finished version of this, good luck with it!