Web cam detection

I wonder if some of you are already using web-cam detection.
I was using laser pointers and photodiods and i was not very satisfied with this.
My doubt is, if I use 30fps (this is a quite ususal setting for video purposes) this means a scan every 33ms, if the car is 12cm long and runs at 6.0m/s it might find it's way between two scans?!?!? is this realistic? do I have to increase the scan rate? according to my calculations I would have to go for 60 or 70fps! to avoid a "slip-through", this might be too heavy for the hardware, can you share with me your experience and comments on this.

I propose you try the demo of web zone trigger here http://www.zonetrigger.com/
if you place the webcam on a slow part of your track you don't need to have 70fps.
I try webzone zone trigger with 20 fps on a 8 lanes track and it seem to work.