Who invented the sign?

Depends what sign exactly.

Some signs (such as modern road signage) was developed quickly after the advent of the car, in order to regulate traffic an artificial system was created and implemented; this has now become an almost international standard. Prior to this there were basic road marking signs attached to posts and poles as well as those carved on large road makers.

Other forms of signage have been in use since man first settled in areas and had to demarcate areas or show direction to an area.

The appeal of visual signage is that it know no language barriers; for example, a toilet sign is basically the same in the UK or China.