Down the Roman Road: Part IV

Question #4: Why are the sidewalks in Pompeiian streets so high?

Stepping stones crossing a street in Pompeii from elevated sidewalks.
Photograph taken by Emily Gilmore, June 2011.

Again, I have read conflicting reports. Most say that the streets are deep in order to contain the mud, muck, and sewage. Others report that the deep streets are for directing and tracking wheeled traffic and keeping it safely away from pedestrians. Concerning the latter there are suggestions that the stepping stones and even the ruts for the wheels are all part of the effort to contain traffic. I have even read that the stepping stones were meant to be speed-bumps, slowing down traffic as it has to negotiate the obstacles.

I believe (like in an earlier post) that some combination of the two suggestions makes sense. I do lean, though, toward the notion that the street itself was an open ditch and that the raised sidewalks and stepping stones are for the convenience of pedestrians.

Bonus question: Are the stepping stones across the streets in Pompeii unique? I wouldn't think that they would be, but I haven't been able to find evidence to the contrary.