Random Thoughts

In rural and suburban towns, why are street signs green? Often times the background of these signs, from the vantage point of a driver, are trees. In the spring, summer and early fall, that background is thus green. Now most drivers who are on the lookout for street signs are those reading directions and/or those who have little idea where they are going. So their focus is primarily on locating and reading the signs, as opposed to paying attention to the road and its surroundings. So wouldn't it make sense for these signs to be in red or another contrasting color so it would be easier for the driver to locate?