Oh this is an interesting and unique concept for the theme. As many others has pointed out, it would be awesome to have a button to speed up the cars. There is many direction you can take this, for example you could add something as subtle as adding different terrain that slows the car down which might cause it to collide with the car behind. Or as Fodi suggested you could add all those crazy wacky thing which will be fun to see hahaha or even both!
Good job and keep it up!