These tourists are showing us how the streets worked--the stepping stones kept your feet dry and out of the rainwater, slops, and animal waste that would have filled the streets of Pompeii. The road itself is rutted with a couple of centuries of cart traffic.
Imagine the streets filled with horse drawn carts, rainwater, human waste chucked from second story windows and horse manure. One of the duties of the Roman officer called an aedile was responsible for keeping the streets clean, helped along by the occasional rainstorm.
Sources
For more on the archaeology of Pompeii, see Pompeii: Buried in Ashes. Also see the Walking Tour of the House of the Faun.
Beard, Mary. 2008. The Fires of Vesuvius: Pompeii Lost and Found. Harvard University Press, Cambridge.
