Where those from wheels, or were the designed for drainage?
According to every source I've read the ruts were cause by iron-banded wheels of uniform axle length wearing into soft basalt. They are deeper in places where the carts were forced to channel the same place as when passing over those stepping stones. The streets doubled as sewers and the rains and waste water ran along the edges (the pavement was crowned). Much more on the roads here
Robert Harris' Pompeii is a pretty good fictional account of the last days of Pompeii. The hero is an engineer, specializing in the construction of aqueducts and the use of concrete. Amazing people, those ancient Romans, and not always in a good way.
Pete: I added Harris' book to my cart and will order when I reach free shipping critical mass.I've been to Pompeii a couple times and there's always something new and exciting there.
In fact, we had some tile work done in a shower using our own pattern inspired by some brickwork at Pompeii.