Radar activated speed limit signs, trigger speed?

[Balmoral Green]

Does anyone know if there are any guidelines as to the trigger speed for these things?

There are several of them in my area, in 30 zones, and nearly every single one of them activates at well below 30mph.

The worst one of the lot is on the A517 between Ashbourne and Turnditch, outside The Tiger public house, Sunday evening I came into Turnditch at 29mph, it went off, I was quite indignant. This afternoon I came in at 25mph, it still bloody went off.

The one at the top of Openwoodgate at Belper also triggers at below 30mph too.

What is the point? If they are going to be triggered by folks who are making an effort, yet they still get a finger wagging, they will just be ignored.

[Gixer]

There is one local to me just ahead of a right turn. Its in a 30 zone and lights up at all the cars including those just creeping forwards waiting to turn right

[Size Nine Elm]

We have one in the local village. It lights up if you walk towards it with the dog.

Bad dog!

[j500ant]

The ones down my way come on at 29 or so. BUT the adjoining council has got signs that just flash 30 in red when you go past at over 30mph. So someone might think their speedo is wrong and the sign says that 72mph is infact 30?