Stepping stones for pedestrians.

These can be seen at Via del Vesuvio, near the House of the Golden Cupids, but you can see many others on Pompeii's streets.
They allowed pedestrians to cross the streets without stepping on the roads, which were very filthy - domestic sewage flowed out there, following the declivity, as well as rain water. Sidewalks were a bit high, to remain above the streets' dirt, and gaps between those stones allowed the passage of carts.