In the 1960s, various levels of government in the Socialist Republic of Croatia initiated planning processes aimed at development of road transport infrastructure of the then constituent republic of Yugoslavia. The first proposal of a motorway link in Croatia was made by Josip Roglić, a member of the Croatian Academy of Sciences and Arts at the time. He proposed the motorway to be built between Zagreb and Split. In 1962, a formal proposal was made by local authorities in Split, resulting in Croatian government adopting a framework motorway development plan in 1966. Designs were commissioned in 1967, extending the route north to Maribor, Slovenia for the first time. The extension largely used the same corridor as an existing Zagreb–Maribor road—Zagorje Highway—a congested and insufficiently safe two-lane road with at-grade intersections, connecting numerous towns and villages between Zagreb and Maribor. The road itself represented yet another upgrade of a transport route recorded along Zagreb–Maribor corridor since Middle Ages. The plan was shelved because of political pressures following suppression of Croatian Spring only to be reinstated upon Croatian independence from Yugoslavia.