Rafael, thanks for the detailed explanation about your 20 UD;
Leighton, these crossings were so common at Pompeii roads supposedly because:
1) they allowed people to not getting their feet wet or dirty as the roads were probably dirty due to lack of an adequate sewage system.
2) pavements/sideways were at least 30cm high in relation to the road, thus for most people these crossing were more confortable to use for crossings.
