Several of the villages round our way have portable warning signs that flash when you enter the 30 limit at excessive speed. These signs are moved around the villages and are usually at a particular village for 4-6 weeks at a time, One afternoon I was on duty, carrying out a static speed check in a village and after 30 mins had not gone one speeder, so packed up and left the village. As we did so we noticed the council had put up the temp speed warning sign that morning !!