In the late 1800s and the beginning of the 1900s important steps were taken to unify transport law starting on the regional level in Europe with carriage of goods by rail with CIM (now COTIF/CIM). For carriage by air the Warsaw Convention (now Montreal Convention 1999) appeared in 1929 and CMR for International Carriage of Goods by Road in 1956. Maritime transport became subjected to the 1924 Bill of Lading Convention (the so-called Hague Rules) as supplemented by the 1968 Protocol (the so-called Hague/Visby Rules). Therefore, I think it is fair to suggest that at the time of the mid- 1970s a global unification of transport law had been achieved although, admittedly, COTIF/CIM and CMR have to be regarded as regional unifications.
Jan Ramberg, Global Unification of Transport Law: A Hopeless Task?, 27 Penn St. Int'l L. Rev. 851 (2009).