To understand what's going on, you need to understand the poor state of Italian traffic regulation. Many streets have no lanes, especially those bordering the major piazzas, and no one bothers to follow lanes anyway. There are cops everywhere, but they seem too busy with cleaning up accidents to enforce the traffic laws. Most intersections in Rome have no stoplights or any signs at all. Drivers entered it at high speed, slowed just long enough to swerve around each other, and sped away once they found the street they wanted.
Double parking is frequent.
This means that you often see cars swerving into your lane from the opposite direction. When double parking reduces four-lane streets to a single lane in the middle, Roman drivers consider that an invitation to play chicken. Roman driving frequently involves merges, because the city is dotted with vast traffic circles. The winner of a merge seems to be determined by aggression, acceleration and mass. In Piazza San Marco, I observed what happened as a huge wave of traffic swung around the circle and encountered another wave of traffic merging in from a large avenue.