The earliest road signs were milestones, giving distance or direction; for example, the Romans erected stone columns throughout their empire giving the distance to Rome. In the Middle Ages multidirectional signs at intersections became common, giving directions to cities and towns.
Traffic signs became more important with the development of automobiles. The basic patterns of most traffic signs were set at the 1908 International Road Congress in Rome. Since then there have been considerable change. Today they are almost all metal rather than wood and are coated with retroreflective sheetings of various types for nighttime and low-light visibility.
New generations of traffic signs based on big electronic displays can also change their symbols and provide intelligent behavior by means of sensors or by remote control. These "road beacon systems" (RBS) are based on the use of RFID transponders buried in the asphalt to allow for on-board signalling and interaction between the car and the road.
Yet another "medium" for transferring information ordinarily associated with visible signs is RIAS (Remote Infrared Audible Signage), e.g., "Talking Signs®" for print-handicapped (including blind/low-vision/illiterate) people. These are infra-red transmitters serving the same purpose as the usual graphic signs when received by an appropriate device such as a hand-held receiver or one built into a cell phone.
What's it worth? Take a look at this Road Sign price guide: sold listings for a value indication.