I suppose it depends a lot on your local area. Where I live there are tons of these 'flashy speed limit' signs. I don't think I've ever seen anyone obey them.

I suppose it depends a lot on your local area. Where I live there are tons of these 'flashy speed limit' signs. I don't think I've ever seen anyone obey them. 35-40mph in the new London '20 zones' is completely normal behaviour if the road permits; everyone does indeed do it, including buses and off-duty police/ambulances. It is, in fact, empirically socially acceptable (by the drivers if not necessarily by people who live there). In areas with speed bumps you see much lower speeds because people don't want to damage their vehicles. Enforcement, basically. The same is true for real speed cameras, people tend to speed up and slow down. That said, mobile phone use is different. I think most people intuitively recognise that using a phone at the wheel, particularly in an urban area, is really quite dangerous, and maybe a sign would work.