How to avoid driving into Historic areas in Italy and Europe in general

Signs are clearly posted at intersections. They're a variation / elaboration of the international 'Do Not Enter' symbol.

Turn without going past them.

Taking the train instead of driving will guarantee 100% that you never drive into the historical center.

If you have ignored or missed the advance warning signs when you get to the final one there is often not a legal way to avoid it.

Don't drive into pedestrianised areas period, unless you are dropping off luggage and have previously arranged said trip.

In Monaco, don't try to drive past the policeman in the middle of the street and drive up the Rock.

Be familiar with all road signs before you drive, and the unmarked laws, such as slowing to 50 kph when you pass the yellow town name sign. That's before you reach the sign, not just as the camera just beyond snaps your smiling face.

You need to know the different speeds for built up areas, rural roads, highways and toll roads in each country as they are all different and it is rare for speed limits to be posted except at borders, and not always there.

For Italy you can assume that just about any town has restricted areas (ZTLs). You can find some information on line, but the best strategy is to go around the towns you will not be stopping at, park on the outskirts or the first public lot you see for the ones you are atopping. Many have noted that by the time you see the signs, it's too late, but those were the people who did get caught.

This bit about driving in France might help, at least for France: http://about-france.com/travel.htm. From what I can see, it looks like now it is even easier to get a speeding ticket than it was for us in 2012.

We didn't drive much in cities, but most of the small towns we went through had digital speed signs that showed our speed upon entering the town in lights appropriate to the quality of our driving. We only saw this once, near Verdun, but my favorite one had no numbers -- just a smiley face, a straight-line grimace, and a frowny face depending on how fast we were going.

Signs are NOT always clearly posted in a way to avoid a ticket. For example in Verona, there's a traffic light that locals with a pass are allowed to use to turn left, but if anyone else does, they're captured on a camera and sent a fine. There is no sign telling anyone this. Yet another reason we avoid renting a car in Italy.

If you ask this question I'm assuming that you intend to rent a car in Italy, at least for portions of your trip.

The first thing you need to do is to get an International Driving Permit at your local AAA.

Secondly you need to learn the International Road Sign symbols used in Italy and the rest of Europe. Here is the list. Print it for easy reference.
http://en.wikipedia.org/wiki/Road_signs_in_Italy

In reference to your specific question this is the sign that indicates the beginning of a restricted traffic zone where motor vehicles are not allowed.
http://en.wikipedia.org/wiki/File:Italian_traffic_signs_-_divieto_di_transito.svg

This is an example of what you see at the entrance of the ZTL (limited traffic zone), in the historical center of Florence. In the case of a Florence they also have a bright red light during the hours of operation (which turns green when traffic is allowed, usually at night)
http://opvakantienaartoscane.info/wp-content/2013/01/autovrije-zone-ZTL.jpg

If your hotel is inside the ZTL inquire with your hotel. Generally you are allowed to enter for the check in (up to 3 hours), but the hotel staff needs to communicate the license plate of your car to the authorities (via website) within 24 hours from your entry.

Rick's introduction to each city or town in his books usually starts with tips for "Arrival in this town... by train, by bus, and by car," as relevant. He will often refer to parking lots which you can then see on Google maps or other resources. Until you learn all the road signs, it's pretty safe to assume that many mean "do not enter."

If you drive in Italy, I would generally park outside the city center and therefore avoid driving within it. That will help you avoid ZTLs. I do love just taking trains and buses there though if you don't need to drive.

In Switzerland, it's obvious. You'll either see the standard No entry sign, or the road will be blocked off. Switzerland has no equivalent of a ZTL.