Again been lying for ages, again a city-wide problem, again needs a sensible management strategy which seems beyond past and current management.
Er, phone all local estate agents to advise / warn would be a good, simple, cost-effective start. (Nah, let's spend £££ on administration and never-ending board pickup instead or just ignore and blight the cityscape). Oh and they need to be told to stop putting the darn things up on ANY public property, else fined, heavily. The prob. being they farm out sign erection to the cheapest 3rd party who don't give a monkeys thereafter ie not paid top remove same.