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Signs at the roadside could eventually be done away with if a transmitter were to replace them. This could be picked up by a micro chip receiver built into cars. The information could be displayed or read out by a computerised voice. For instance you have just entered a 40 MPH zone.
This could be encouraged by the friend of the earth groups, think of the tons of materials that could be done away with. Also when the signs needed updating this would be a simple download process from the traffic authorities.