Very nice game! I can tell that a lot of work (and time) went into the game!
Now for my suggestions:
1. Add a passenger seat on a car, so that it people are playing together, they can travel together!
2. Add a speedometer, or some sort of speed indicator, when you are driving a car.
3. Add some sort of boost pad that you can place on the ground and on ramps. Currently, you are not able to drive up the ramps (if it is possible, let me know how!), and a strategically placed boost pad would do just the trick!
If you have any questions about my suggestions, or want me to explain further, let me know! Thanks!