Traffic Sign advertising

This is only for temporary advertising, you will see that every road sign a blank back with nothing on it,the idea i have is, paste a few posters on the back of the traffic sign an make sure that the letters are big in bright cause the advert will be in the opposite side of the road where you are driving so it must be clear and visible for the driving eye,there is a lot of signs out there so have fun,make it easy to remove posters so that if council goes out to clean they can easily remove it,cause i not sure if this is legal,but i have seen a lot of advertisers to use this strategy.