I agree with Marko that it would be better if the cars were sharper. It takes a lot of practice (and it is not easy) to try and match your panning with the speed of the cars. I was at Mosport this summer and spent about 3 hours (and probably 300 to 400 shots) practicing panning to get the panning motion down and the shutter speed to where I wanted it. I finally zeroed in at 1/100 sec as the optimal shutter speed for my panning experience. With more experience you could probably reduce the shutter speed to maybe 1/60 or less. In fact I saw a video podcast of an F1 photographer and he can pan and get sharp pictures amazingly at 1/15 sec. Here's a link to my pictures;

Vintage Cars - Panning Practice