My mom's been wondering if I could find this story but I really can't without more specific details than she can give me. It's rather old, she guessed somewhere in the 1960s or 70s. The story wasn't centered about the science fiction aspects from what she remembers, it was more of a murder mystery or detective story that involved a lot of science fiction elements.
There was a highway that was made of a heavy, green glass that was uncovered in the sand in Australia after the technology to create it has been long lost. People use it to get from place to place extremely fast, e.g. going from Perth to Sydney would only take an hour. The author described Australia in great detail, and she recalls that the descriptions were rather awe-inspiring and made her want to visit because of how beautiful it sounded.
More on the highway was that they could repair it but couldn't build more of it due to the loss of technology. The main character (some sort of detective?) spent a lot of time on this green glass road going places. They also had taken - chipped off - hard chunks and studied them but couldn't reverse-engineer the highway or something like that. The highway worked off of something like magnetism, like having the glass somehow be magnetic.