That's a brilliant approach. I have to assume that if cars are using this, roads could be doped with materials to create an even stronger signature. Once a road was paved, it could be scanned, and then the results sent to auto-companies for addition into their database.
There are a lot of approaches that can be taken with this system since the signature can be arbitrary. Actually having a unique fingerprint for each stretch of road would give you the option to navigate specific places differently without the need for active GPS. Surface markings could be turned into bar codes with the addition of metal strips. :)
If you're going to be embedding things in the road, might as well go all out and use passive RFID tags, so you can also include some hints about local regulations ("this is a school zone") that you might not want to rely on Google keeping track of.