Commenting on the posters, a Hertfordshire Police spokesman said:
We are not saying it is going to stop hardened criminals but it may make someone who is nervous think twice.
Yes, they might think twice. After which they'll probably go ahead and do it anyway, because, so far as I know, a sign does not have powers of arrest.Policemen, on the other hand, have extensive powers of arrest. Maybe, when they've finished putting up the signs, they could actually try doing the job they're paid for, and go out patrolling the streets. Somehow, I think that the sight of a burly policeman watching them with interest might prove a rather better deterrent for most aspirant criminals than a sign telling them not to be naughty.