Here's how to file a complaint against an Istanbul taxi driver:
How to Avoid Unpleasantness
1. Have your hotel call a taxi from a local taxi stand so that you have someone to complain to if service is not acceptable. Ask at the hotel what the fare should be.
2. Write down the taxi's license number before you get in: there are license plates at the front and rear, and the number is also painted on both side doors and on the roof.
3. Check the meter during your ride, especially when you are arriving at your destination. Double check the money you hand to the driver in payment, look into his eyes and speak the amount: "Twenty liras!" Watch what he does with the money.
4. From either of Istanbul's two airports (Atatürk/IST, or Sabiha Gökçen/SAW), use Backpackers Travel's excellent private transfer service in a comfortable Mercedes-Benz minivan that costs about 40% more than a taxi (but may cost less if the taxi driver overcharges you!), yet is 1000% more pleasant, safe, comfortable and assured—great value for money. More...
6. Use public transport! (Tram, Metro, Bus, Tünel, Füniküler, Sea Bus, ferryboat, suburban train.) In fact, Istanbul traffic is now so heavy that travel by public transport is often faster by tram and Metro than by taxi. (Here are the city's traffic webcams.) Public transport is frequent, reasonably comfortable, and a one-hour, 15-km trip all the way across the city need cost no more than the first 90 seconds in a taxi.
How to File a Complaint
The license plate number of each taxi is painted prominently on both sides and the roof of each taxi. Note the number, date and time, and location(s) of the incident and send a description to the Istanbul Public Transport Services Directorate: