My wife and me will be in Tuscany the first week in April.First visit in this region of Italy.Renting a car in Florence,but staying in my cousin's farmhouse 15 mi.west of Siena.
Approximately what percentage of the towns there have ZTL warnings that I have to make sure I don't drive 'past the point of no return?'
If you search through the posts, there is a list that was compiled of known ZTLs, though certainly not all. For all practical purposes, figure that any town worth visiting seems to have one, plan on parking in the public lots on the outside of town and bus or walk in. For Florence, take the train or bus in from a smaller town.
Here is a good website with introductions to driving in Italy incl. info on ZTLs and road signs:
http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm
Assume that pretty much every town will have an area in the inner city that is reserved for pedestrians. It's not always called a "ZTL" and not always controlled by satellite but you'll risk a ticket nevertheless. The main sign to look out for is this one:
It may come with additional signage detailing in Italian the exceptions for those allowed to enter. With the additional symbols it's not that difficult to read even if you don't understand Italian but if you only have a fraction of a second to glimpse at it and are in doubt err on the side of caution.
In addition to ZTL's, you should be aware that there are speed cameras everywhere, not just on the autostrada. They are very common in the area you will be driving. Go the speed limit and you will have no problems. (we have been home almost 2 years, so far so good.) There will be a warning sign and then you pass a box with camera. We met a tour guide in Florence who was from California. Her father had driven in the area a couple of months earlier and had received 6 speeding tickets at that time. (She figured he would be getting more) We stayed in Greve and I lost count of how many cameras we passed driving to Siena. Go the speed limit, especially as you enter small towns.
This is the post listing towns with ZTL's
http://www.ricksteves.com/graffiti/helpline/index.cfm?topic=28422
The above post is correct: this list includes only the towns we've heard of having ZTL's, there are probably others.
Yes, MANY other towns that have ZTLs. Even if they are not car-free pedestrian-only areas, many small towns only allow local residents to drive into the historic center. I noticed this is common in the walled hilltowns near your homebase. For planning purposes, you should expect that parking on the periphery of town to be the norm. I've been to non-tourist towns where you can drive and park in city center but many of the narrow, twisting medieval streets in the residential section may be restricted to local traffic only.
Lucky you! Your cousin's help will be invaluable.