There are tour buses which take you past all the main sights in a couple of hours, and give a recorded commentary in a choice of languages, so I got on one of these this morning. I'd have fried on the open top deck, so I went downstairs hoping there would be air conditioning, but there wasn't!
When the bus was moving, there was a pleasant breeze through the windows, but much of the time we were sitting in traffic jams. This also meant that the commentary kept getting out of sync, so the whole thing wasn't an unqualified success.
Fortunately, because you can get on and off thd buses to visit the sites (or sights!) you buy a ticket for 24 hours, so I did the trip again in the early evening, when it was cool enough to sit on top. It was interesting that there seemed to be less traffic in what we would think of as rush hour than there was mid-morning, which is usually about the quietest time in British cities.
I'll get my money's worth out of the ticket by using it to get to St Peter's tomorrow morning. I'll then have to work out how to get back, as it will have expired before I've seen everything around the Vatican.
