Down the Roman Road: Part II

Question #2: Why are there obvious ruts in Roman roads?

Ruts between the stepping stones in Pompeii

The quick answer is that these ruts are obvious signs of wear and tear. Not so fast, though. Often the quick answer is not the correct one. I have come across conflicting information about those ruts found in the ancient roads in Rome and Pompeii. Some sources do say that they were worn by the continuous passage of wheeled traffic. Even the more sensational sources like to say that they were left by chariots! (Chariots? Hah! That will have to be another post.) These sources claim that the ruts became prominent particularly as traffic edged between those large stepping stones which allowed pedestrians to cross from one side of the street to the other without stepping in something unpleasant. Other sources say that the ruts were carved deliberately in order to guide the traffic more easily between the stones or around certain curves or other obstacles.

Which answer is true? Probably some combination of the two. The problem is that I can't seem to find a definitive source for any of the claims. I would appreciate anyone who can point me in the right direction on this issue.

Something else to consider is that the ruts can be found elsewhere in the roads than between the stepping stones. I believe that they are intentional and that they helped to guide the carts and wagons along the road without incident. While the ruts, or let's call them "tracks" (like a railroad), are useful for wheeled traffic, they do make the road more difficult and dangerous for the humans and animals which pulled/pushed these carts and wagons.