In recent years the road network is much improved, especially the secondary roads, which are almost all paved. The Corinth-Tripoli highway and the Athens-Patras are the key access roads.
It will still take a lot of time to travel in Peleponnese in spite of being almost as big Lombardy.
The road signs are written in Greek and Latin characters, at least on main roads. What becomes more complex when travelling in more remote villages: one more reason to finally learn the Greek alphabet!
You will also find many dirt roads, especially when you reach some beaches off hand or some remote villages. Use caution especially at night.
Buses are more preferable and are more comfortable and much better organized.
Buses are also the key means of transport used by the Greeks.
The company is called KTEL and their color is green; as we go in the provinces their condition is not always good but improvements were maid because of the Olympics in 2004.
The tariffs are not very high, and are around EUR 7 per 100 km.