ASCs pass the information from the first camera to the second, from the second to the third, and so on. That way, the average speed of each section of the road is monitored separately.
Numbering the cameras along your route as 1 to 4, in order, and assuming that cameras 2 and 3 are positioned at the two ends of the 30 mph limit:
Cameras 1 and 2 will combine together to check your average speed over the first (rural) stretch of the road.
Cameras 2 and 3 work together to check your average speed in the 30mph zone.
Cameras 3 and 4 are used to check your average speed in the second rural stretch.
If Cameras 2 and 3 are not located at the start and end of the 30mph stretch, there must only be two separate lengths of road which are monitored. i.e. between Cameras 1 and 2 and between Cameras 3 and 4. I can't think of any circumstances where Cameras 1 and 4 would work together (as suggested in your post).
Chris