I cannot see a truly comprehensive speed signage happening. In order for this to happen you would need a speed limit sign just after every junction on every road. You could correctly argue that a driver should know what the speed limit is of a road, but exceptions do occur where a limit is lower than you believe it is or should be.
Back in 2007 when in south of France the local area we were in had on trial a coloured line system at the side of the road. The colour signified the speed limit. Excellent idea in residential type areas but I don't recall seeing it since.
Colin