I liked the game! Played three times on Easy, got 20 score after one lost game.
It indeed is a very clever idea! I suppose this game can train your attention quite nicely, especially if playing on higher speeds/difficulties.
One of the commenters told you that you can also use the same concept of choosing cars but with letters instead of numpad. It's a clever idea, and I think you can give the player a choice before playing if he wants to use letters or numbers.
First time playing you do not understand when the cars move. Idk if it has some solution besides a tutorial, though in general you can show car's path when certain car is chosen, e.g. if you choose the car with number 3 you will see transparent arrow that shows the path this car is following. This would be nice for planning ahead and not getting confused.
Funny SFX, nice soundtrack, cute cars.
Overall, the game is very nice! If you add more content into it, it will be even more interesting!