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.
To be honest, I have no idea how they (i.e the councils) select their trigger speed, it's more than likely that they do set them lower by a percentage because the indicated speed on your speedo will be higher than your actual speed. It's also possible that they set them lower to provide some buffer with regards to the cosine error (which is always in the motorists favour) so that a slightly incorrect installation will still be spmewhat accurate.
All I can tell you is that while we were developing this and its companion doppler radar board, we got some long stares from motorists as they drove past us!
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