method where commuters load fares on a thin plastic card. Also, the private carriers were introduced and integrated in tariff system – Integrated Tariff System (ITS). There are 145 lines, out of which 12 are tram, 8 are trolleybus and 125 are bus lines. Service operates daily between 4am and midnight, with a limited night bus system. GSP also operates school bus lines, and transportation for the disabled. The entire traffic grid is divided in two zones. As of 2017, GSP Belgrade has 1,582 vehicles in operation. Most of GSP's revenue is still generated through subsidies by the city of Belgrade; in 2017, that amount stood at 65 million euros. In April 2019, GSP along with the city of Belgrade signed a contract to purchase 244 new buses, of which 70 are manufactured by Turkish BMC and 174 by Chinese Higer. In october 2021, GSP signed a contract to purchase 100 CNG buses with Turkish BMC. Vehicle coloring Until the beginning of the 1960s, the vehicles have been colored by cream and red color scheme (buses have been colored until 1961. trolleybuses and trams are colored until 1964). From the beginning of the 1960s, the vehicles have been colored by Mussolini livery – light and dark green color scheme (for buses from 1961 to 1967, for trolleybuses and trams from 1964 to 1970). From the end of the 1960s, the vehicles have been colored by cream and green color scheme (for trams and trolleybuses from 1970 to 1979, for buses from 1967 to 1987). From the end of the 1970s, the vehicles have been colored by cream and red color scheme again (for trams and trolleybuses from 1979 to 2003, for buses from 1987 to 2004). From the beginning of 2000s, the vehicles are coloring by three color schemes with blue bottom (trams are colored by red and blue color scheme from 2002, trolleybuses are colored by orange and blue color scheme from 2003 (for some units, but their current color scheme is red from 2010), and buses are colored by yellow and blue color scheme from 2004 (in future, their color scheme will be red from 2015)). Some vehicles have been colored by cream and blue color scheme (like the private buses in the end of the 1990s, for example: one trolleybus ZiU-9 and one tram ČKD-Tatra KT4YU have been colored in cream and blue color scheme in 1993