Organized roll racing??

My brother and i were watching that last night, 60 rolls on an air strip.. the confusion was how do they know they left together?
 
It looks like they were using a traffic light to send 'em off. So, the question isn't necessarily when do they know to go. It's more who's making sure that they're not going faster than 60mph when the light goes green? In other words, if guy A is going 60 and guy B is a few car lengths behind and speeding up to get side-by-side....who's going to make sure he slows down to 60 before the light changes?
 
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