The best way to get to Kocaeli is to fly to Istanbul Sabiha Gökçen International Airport.There are Havaş busses from Istanbul Sabiha Gökçen International Airport to Izmit every two hours.
ISTANBUL ATATURK AIRPORT (http://www.ataturkairport.com)
SABİHA GOKCEN AIRPORT (http://www.sgairport.com)
By Air (Via Istanbul)
The Turkey's busiest airport, Istanbul Ataturk's Airport is only 110 km to Kocaeli. Istanbul's other airport, Sabiha Gokcen is around 50 km away.
Landing to Ataturk's Airport you can take the subway system from inside the terminal building. Follow the signs of "Metro". In the subway there is only one line and only one direction, no possibility of losing his/her way. With a 20 minutes ride reach ESENLER station. In Esenler you should change to a special train going only to the Bus Terminal approximately in two minutes. Esenler Bus Terminal is said to be the World biggest bus terminal, this claim can be true so don't be surprised by the size and the complexity of the building, try to find below mentioned bus companies:
Efe Tur (www.efetur.com.tr),
Asya Tur (www.asyatur.com.tr),
Gurkan Seyahat and Kocaeli Seyahat
These companies have regular services in every half an hour to Kocaeli (Also regularly called Izmit). The service takes 90 minutes to reach Kocaeli center (Halkevi Station)
Landing to Sabiha Gokcen airport makes you much nearer to Kocaeli, but there is no direct public transport. You can take either regular buses of HAVAS to go to Harem Bus Terminal (In the Asiatic side of Istanbul). From there you can look for the same bus companies mentioned above. From Harem it only takes 60 minutes to the center of Kocaeli. Or you can use your bargaining skills with the taxi drivers waiting in front of the airport's terminal. Although all the taxis are equipped with proper counters, for long distances it is not surprising to settle the price beforehand. 80 to 100 TRY should be a correct price for direct ride to Kocaeli center.