We did this last week. Isn't this an awesome idea for a kid who loves cars? Well Henry liked it but he wasn't super impressed and he was done in about 5 minutes and it took longer than 5 minutes to clean the paint off the cars. We had done something similar months ago on small paper which he did really like.
I just spread the paint out in lines and the cars drove through them and made tracks. Some of them even made rainbow tracks after driving through several lines of paint which I thought was awesome.
I am up for trying again in a couple of months and hoping for a little more enthusiasm from that boy.