Given what we know of Roman engineering prowess and their money-grubbing practicality, one must wonder why modern academics attribute the ruts to roads wearing out, and with equal dedication, that the Romans refused to bring the roads up to standard. Pompeii was the playground of the rich, in its ascendancy before it was cut down and buried. This argument holds no water.
I have a much better idea: A small layer of fine sand in the bottom of those ruts would make for a very smooth ride (comparatively) and would also control the clatter of metal rims on hard stone in the narrow streets. The ruts could have easily provided noise control and protection for Roman wheels.
The recent discovery that the Romans filled the cracks between their paving stones with molten iron or ceramic powder suggests the Romans were making incredibly expensive, high tech repairs to the smallest defects of their roads instead of what we would consider the largest.
Noise abatement is luxury--- a common Roman demand. And how can one expect the Romans, who have quite primitive metallurgy, to maintain a civilization if they are always repairing their limited, expensive vehicles due to driving them over hard stone streets??
If you have ever listened to a horse drawn carriage over rough cobble stones, then you have done so in the modern era. These things would normally be covered with mud, straw, or worse. A small spongy coating would protect the horses, the wheels, the people, and prevent everyone from going insane due to noise. I suspect the Romans, as usual, took it to another level of excellence.