whilst I'd like to think I don't speed generally (most certainly not in a built up area) as I use the speed limiter function on the car almost every journey to prevent that (i.e I set is at 32mph in a 30 zone as my GPS reports that's between 30 and 31 mph), but I have nearly been caught out a few times on unfamiliar roads.
One was quite recently where I was in a national speed limit area (so 60mph) and was doing around 55 (as it was a twisty road), then after rounding a bend, there was a 30mph sign (no 300 yard markers and such), and 20 yards after the sign was a speed camera. I had to brake quite heavily to get the speed down (which the car right up my chuff behind me was non too impressed with), so having some sort of warning in the car would have at least better prepared me for it.