Yes I think they do have a
Yes I think they do have a higher threshold plus as Purplecup says they work in a different way so are inherently harder to set off if you are on a bike – that said, they catch motorcyclists who’s radar shadow can’t be that much bigger than a cyclists.
One thing I have noticed with the Slow Down! signs is that they seem to be very sensitive some times you can get them to flicker slightly when you don’t seem to be going that fast, and I’ve certainly noticed that I’ve had them flash at me in the car when the speedo has been reading much closer to 25mph. Dunno about you I always try not to set them off when I am in the car and always set them off when I am on the bike.