They are designed to attract the fleeting attention of drivers - and as such, are a menace, apart from being a blot on the landscape. But five words are hardly enough to deploy any kind of meaningful argument. So a poster cannot be expected to do more than attract an enquiry, on its own. This is how, for example, La Redoute, the big French mail order firm, uses them to generate catalogue enquiries. We find them very effective in attracting enquiries on the telephone.
Diamonds are nothing more than chunks of coal that stuck to their jobs.