I must be looking at a different section of the road, I recall dual carriageway with 2008 images when I looked.
Unfortunately, thanks to poor maintenance, poor management and local authorities messing about, it's difficult to look at images of sections of roads to clarify speed.
A dual carriageway has a national speed limit of 70mph for cars, I would say 85 percentile speed is at least 60, so no wonder they are raking the tickets in.
Breathe, I'd guess 80% of 20MPH limits are unenforceable again, see above.
I wouldn't expect speed traps on these roads not cost effective enough.
Reducing speed limits has no effect on road safety or average speed on that section, local authorities see it as a cheap fix.