There are no fees to use the highways except intercity motorways (otoyol). While Turkish highways vary widely in quality and size, the toll motorways have three lanes and are very smooth and fast. Motorways are explicitly signed with distinct green signs and given road numbers prefixed with the letter O. The motorway network currently consists of two lines stretching out to east and west from Istanbul (towards Ankara and Edirne respectively), a network in Central Aegean fanning out of Izmir, and another one connecting the major eastern Mediterranean city of Adana to neighbouring cities in all directions.
Motorways no longer have toll booths and instead have lanes for automatic (OGS) and pre-paid card (KGS) lanes. Unless you are going to live in Turkey, the refillable KGS card will be what you want.
The KGS cards can be purchased in a building at the entrance to the motorway. The building may be on the other side of the motorway in which case you will have to park and cross the motorway on foot. This building will have a window for purchasing a KGS card and putting money on it.
Once you have purchased a KGS card and put money on it, drive through the KGS entry lane to the motorway. Place the card in the scanner machine which will display your card’s current balance. Upon exiting the motorway, scan the card in the same way and the machine will deduct an amount based on the distance driven.
It is also possible to purchase KGS cards at several Turkish banks.
In addition to the distance driven, motorway fees also depend on the type of your vehicle. Edirne–Istanbul motorway—about 225 km and the main entry point to Istanbul from Europe—typically costs 3 TL for a car, for example.