There are loads in the UK and have been for years - green numbers if you are doing speed limit or lower, red numbers if you are speedingOoooh...fancy!
We have the ordinary ones - that just show the speed in red - in Houston. There is a city-within-the-city that is a notorious speed trap and they put these up a few years ago. I was surprised they were willing to give people a head's up; that neighborhood has the cleanest streets and most well maintained sidewalks and buildings in the city off the revenue they make on traffic stops.
Bet you're talking about West U. Because that's exactly what I was going to post about. I've never seen temporary ones! Just the permanent ones. They tell you how fast you're going and flash if your speed exceeds the limit.
It is actually a notorious speed trap, but I believe that they really just don't want people speeding through their bedroom community which is smack in the middle of high-speed Houston. It has a few major thoroughfares crossing through and the speed limit drops from 35 (read:45!) to 30 (read:30!). In other words, the message seems to be: you can speed all you want in Houston but it's 30 here and we really mean it.
I actually got stopped there once, not for speeding, but for something else which I was totally guilty of but unintentionally. Cop (rightfully) stopped me, and after I readily admitted my guilt, he checked my license and insurance and asked me a bunch of questions about where I was going, etc. Then he let me go! Maybe it was because it was obvious I'd made a stupid mistake and wasn't just trying to get away with something.