That’s a yellow brick road, and the reference is of course to The Wizard of Oz.
Terry’s pun also reminded a correspondent of an old joke about an Oz frog with a bright yellow penis who hops up to a man and says: “I’m looking for the wizard to help me with my ‘problem’.” The man answers: “No problem, just follow this road until you get to the emerald city.” The frog thanks him and hops off along the road. Shortly afterwards, Dorothy and Toto come along and she also asks the man where she can find the wizard, and then he says: “Just follow the yellow dick toad”.
Well, I thought it was funny.