All very interesting.
I think I will be having a few problems with many bikes bunched together and multiple triggers. And then there are the front wheels and the back wheels. I could end up having about "triggers happening at once".
I had originally thought of calculating the average speed of the bikes and triggering it by hand or with one beam nit that is a bit of a cop out. I have already started on the PC end. THe system runs from midi and we treat it like a "visual" musical instrument. For anybody who is interested there is a link here to a very bad mpeg of a brief snippett from our show at Alton Towers. We developed moving platforms for the flames which you can see at the front. To get an idea of scale, our flames are about 24' high and the background is Millenium fireworks + lasers etc.
http://elmtreecottages.co.uk/Alton1.AVI
(I'm afraid it is about 24 meg)