Well that may or may not be the case on the mainland, but I can tell you in NI the sites for safety camera enforcement are allocated by criteria based on the number of speed related fatalities on that section of road. The majority of the sites are on 50/60/70 mph roads.That said, only a particularly naive observer would believe that, at the highest level, speed camera enforcement is designed to keep us safer and not generate revenue.
A mere cursory glance at the pattern of enforcement reveals otherwise. There is a tendency to target big, busy roads where it's easy to creep over the 30mph limit. Of course, these aren't the roads people are dying on week in, week out.