"A dream of the people and of the country has been achieved," Turkey's Prime Minister Recep Tayyip said at the ceremony formally opening the Istanbul-to-Ankara route of the 2.8-kilometer (1.7-mile) tunnel crossing Bolu mountain, half way between the two cities.
Erdogan drove Italian Prime Minister Romano Prodi through the tunnel, which was built by an Italian company.
Turkey hopes to open the Ankara to Istanbul part of the project in the spring. The project also includes a 4.6-kilometer-long (2.8-mile-long) viaduct and bridges that bypass Bolu mountain, which is difficult to travel over in winter. Until now travel time between Istanbul and Ankara was five hours, reports AP.
Work on the tunnel began in 1993, but construction was delayed because of a lack of funds and an earthquake in 1999 that devastated the Bolu region.