The ruby slipper, as used in The Wizard of Oz, was originally supposed to be silver. This was, of course, ruined for the film version, which lent a certain ambiguity to Baum's original purpose for the slipper.
The silver slipper and the golden brick road were both representative of the United States Treasury issue of the day: whether to keep the gold standard or to include a silver standard. That is, paper money would be backed by silver as well as gold.
The largely accepted interpretation of this reference is that he was supporting the addition of silver to the Treasury system.
The silver/gold reference was one of several depicted by Baum, including the similarity between the characters and their real life counterparts, as well as the Emerald City being the symbol of a happy and healthy currency at the end of the road. See The Wizard of Oz.