I found myself in that unfortunate position a few years back - having driven from Aberdeen down to Glasgow , by now late at night , and dark , my fuel light came on , so I turned off the M8 to go to Sainsbury’s which I knew was nearby .Doing 40 in a built up area with parked cars and pedestrians (including kids) around is obviously not sensible or clever. But it doesn't follow that the same behaviour is equally dangerous on all roads with a 30 limit. There's a fixed camera near me (on a busy A road) that records a HUGE number of offences per year - over 19,000 in 2017 - without there ever having been a fatality or even serious injury on that stretch of road. In fact over a 5 year period the average was only 3 accidents per year - all recorded as 'slight'. So while exceeding the 30 limit is still potentially fatal (and illegal of course), a huge number of people regularly doing so there over a long period of time doesn't appear to have had any significant impact on road safety.
A week or so later a FPN dropped through the letterbox, indicating that I had been pinged at 42 in a 30 , something I would never intentionally do .
The street name meant nothing to me , so I looked it up . This was an urban dual carriageway ( the one from Ibrox out to Renfrew , and just passing Luma Tower ) which had been a 40 limit as long as I could remember - but when I returned to the scene of the crime , the 40 signs ( previously it had gone from 30 to 40 just after the roundabout ) had been removed and there were little yellow ‘new speed limit in force’ signs .
A further visit at night , as it was when I was caught , showed that the road was full of parked vehicles , including vans big enough to obscure the signs .
Not worth arguing about though and I just paid the FPN without complaint . One insurer just said thanks for letting us know , and the other charged a £25 admin fee . Neither bumped their prices the next year .