That looks like one of those speed-sensing warning signs you find in built up areas and accident blackspots around the UK?

If so, can you answer this: Are they typically set just below the speed limit? I.e. 29MPH in a 30MPH zone? My reason for asking is that I am very careful to keep to the speed limits in residential areas, typically 30 and 40MPH zones, and even then a lot of them flash up the speed limit (not the actual measured vehicle's speed) seemingly to chastise me for speeding! With one in particular that I go past every day, I know that if I go past it at 25MPH it won't illuminate.