The Metrobus is a 52 km (32.3 mi) bus rapid transit route in Istanbul, Turkey. The system has 44 stations that follow the city's ring road via Avcılar, Zincirlikuyu and the Bosphorus Bridge to Söğütlüçeşme using dedicated bus lanes for almost the entire length of the route.

The M4, officially referred to as the M4 Kadıköy - Sabiha Gökçen Airport metro line, is a 33.5-kilometre (20.8 mi), 23-station rapid transit line of the Istanbul Metro. It is colored deep pink on the maps and route signs. Running between Kadıköy and Sabiha Gökçen Airport, it is the first rapid transit line operating on the Asian side of Istanbul. The M4 mostly runs under State road D100, parallel to the Istanbul-Ankara railway and is entirely underground.