Down the Roman Road: Part III

Question #3: How in the world did traffic navigate those stepping stones in Pompeii?

In my last posting I considered the ruts between those stepping stones in Pompeii and pretty much came to the conclusion that they were deliberately carved in order to direct the carts and wagons between the stones and elsewhere along the route. My question above is directed not at the vehicles themselves but to the mules, donkeys, horses, oxen, and humans that propelled them. Yes, the wheels slip nicely into the ruts but how did one donkey (let's say), bound to the front of a wagon, avoid the stones? He couldn't step around them with the wagon "in the groove" nor could he step over them. Even if he were to step around the stone, how did he not damage his hooves, feet, or legs walking in or over the groove itself? This question tends to give my notion some credence that the grooves were filled with sand or other substance. The same question becomes even more interesting if the wagon was pulled by two oxen. They must have walked been trained to walk between the stones AND avoid the ruts. Furthermore, a cart pulled or pushed by a human would have the same difficulties. Any enlightenment anyone could give me would be greatly appreciated.