Not bad Lennart, not bad. But I'd recommend using the blur more wisely instead of randomly. There's blur here and there, no matter how distant or close.
Real blur mainly occurs real close to the lense and far from the lense. If you'd like to focus on the car, you gotta make the part of the track wich is as distant to the lense as the car is to the lense clear, and blur the more the further and closer to the camera.
I can tell you all about how to do this but DoN already wrote a great tutorial for distance blurring, DOF Blur.