Penny, as discussed previously....
SO2296 : Beware toadsYour picture was taken in August, when there is no danger of flattening toads - they only migrate in the spring. Round our way, the sign goes up in February and comes down in May, by which time the migration is over.....
SO7642 : Toad warning sign (managing toad crossings is a serious business - the link tells the full story)
The other reason for taking the signs down is that visitors tend to steal these amusing signs!