JR and I rented scooters last night and took a quick moto tour of Rome, which also gave me an opportunity to experience Roman traffic fifs-hand. Thanks to prior experience on a race track it wasn't too hard to get used to, but it bears little resemblance to driving in the US or Australia.
Unlike the US, there are no lanes. Every square inch of pavement is fair game. Whoever is in front has the right of way, so if you can get your nose in front of the other vehicle. There do not appear to be any speed limits other than prudence, however Rome is congested enough and there are enough pedestrians and obstacles to effectively limit speed to about 50mph. This is just a guess, I didn't look at my speedo at any point during our ride.
That said, there are rules to driving here. I'm not sure how they are enforced; certainly the Carbinieri are far too busy smoking cigarettes and looking cool in their Armani uniforms to do anything as prosaic as write tickets, but nonetheless rules are followed. Pedestrians are yielded to, although only if they walk in front of your vehicle, red lights are obeyed, and cars do not drive in the opposite direction lane (some fudging is allowed on that last one).
Traffic is heavy during commute hours but there are lots of back streets and alternate paths. You can get stuck on back streets because they are frequently only one car wide, and parking in the middle of the street (thus blocking the entire street) is fair game for delivery trucks, or anyone wishing to have a quick chat with a friend.
I gave the scooter back at the end of the night unharmed, and with no perceptual gas usage. Fun trip!