Parking and Driving in Sicily are in the main pure hell on earth in varying shades of angry red. The only real exception being traveling the Autostrasse where the road is wider and volume of traffic much lighter. I'll try to recount the visions of hell in order of ascending hellishness below.
Vision of Hell #0.5: Autostrasse tolls.
Not a big hell, just a bit of an irritation. The A20/A18 between Palermo and Catania has the most tolls, and the whole trip will cost you about 12 or 15 euro in tolls. The A19 through the middle, on the other hand, has little or none. Either way its only a little hell because the driving is easy. One thing to watch - the A20 between Palermo and Messina is not finished and there is a long stretch not long after Cefalu where you have to follow the single carriage coast road. It's quite tortuous as it twists and turns, and because it's a major freight route, you may well find yourself at the back of a slow moving convoy for mile after mile.
Vision of Hell #1: Parking in small/medium sized towns.
This is only a minor hell, punishment only maybe for a few lies. Taking Enna and Giardini Naxos as examples, you'll find parking a problem. Dedicated car parks are rare and usually you'll have to take your chances on the (usually very narrow) streets. At this point, you'll need to watch out as it is generally not free and parking enforcement (ticketing in other words) is probably Sicily's most efficient public service. Charges will vary from about .50 to 1 euro per hour, and you'll buy your ticket either from a patroling, blue-vested Poliza Municipale or sometimes the bars and Tabacs sell them.
Vision of Hell #2 - Driving round trying to find things.
Oh Lord, please give me a sign!!! This is a step up and punishment for something more sustained and substantial like theft or arson. Don't expect notable attractions to be well posted, easy to find, or easy to park your car near to. Taking Monreale as an example, I drove around its extremely narrow, vespa-infested streets for as long as the tears held off without locating The Duomo or Cloisters. In the end, the trip had to be aborted and I spent another 30 minutes just trying to leave in the right direction. This is common, even in places you expect to be a bit more laid back like Monreale (described in many guide books as a "hill town") and Enna (ditto). Generally, places will be more crowded and hectic than you expect.
Vision of Hell #3 - Driving manners.
The ultimate punishment in terms of its severity. Let me describe the road manners - there aren't any. You can be overtaken at any point by anything on any road on either side of your vehicle. Vespas are the worst - they just decide to overtake and it's your job to make space, not theirs to decide if there is any! You will never EVER be flashed in to a line of traffic, not even by a little old lady or a priest.
Vision of Hell #4 - Driving in Palermo.
This is indeed hell in a very, very angry shade of red. Basically, it's visions 1 to 3 added together and multiplied by 10. I am crying as I recall the two hours spent trying to get out alive. Why was I there in the first place? By pure accident - a wrong turn on the way back to the airport to drop the hire car off. But once you're in, you're in. Vespas are everywhere, signs are nowhere. You arrive at a crossroads (most crossroads) and there are NO SIGNS telling you what is in ANY direction!!! So you just have to guess. If I'd seen a sign for Rome or Prague, I'd have followed it - at least I'd have known where I was headed. Double parking is commonplace - people just stop their cars in a line of traffic and wander off!!! I will NEVER, EVER do this again, even by accident.