Brotherhood and Unity Highway A modern highway connecting Zagreb and Belgrade was originally designed in the late 1940s, and construction began on April 1, 1948. A substantial portion of the work was carried out by youth from
Yugoslavia and abroad through
youth work actions. Nearly 300,000 youth, including
Yugoslav army soldiers, took part in the construction. The road spanning the two cities, called Brotherhood and Unity Highway before the 1990s, was completed and officially opened on July 27, 1950. Despite being called a motorway (), it was a single carriageway, two-lane road with all of its intersections at-grade. Due to a shortage of construction material, the
wearing course was executed in various materials—mostly concrete slabs, with some
asphalt concrete sections and even some
cobblestone-covered sections. The road was originally designed to carry approximately 9,000 vehicles per day, and the original construction plans provided for subsequent conversion of the road into a four-lane motorway. In 1971, as the road gradually became congested, the plans to build the motorway on the route were approved by the
Parliament of the
Socialist Republic of Croatia. Unlike the other two motorway plans approved by the Croatian government, Zagreb–
Split and Zagreb–
Rijeka, which were cancelled after the suppression of the
Croatian Spring, the Zagreb–Belgrade motorway plans remained in place and an expansion of the existing motorway into a dual-carriageway, six-lane motorway with exclusively grade-separated intersections proceeded. The additional carriageway was to be built south of the existing road, which was in turn to be widened to accommodate two traffic lanes and an emergency lane.
Prewar construction and wartime use Construction of the A3 motorway in Croatia in its present form started in 1977, when the first six-lane (including emergency lanes) sections were constructed as a part of the Zagreb bypass. In 1979, the section connecting the Jankomir and Lučko interchanges was the first to be completed. The section between the Lučko and Ivanja Reka interchanges was opened in 1981, marking the completion of the A3 section of the Zagreb bypass. Unlike the remainder of the motorway, which was constructed by widening the existing road and adding the additional carriageway, these two sections were built entirely from scratch. The first part of the six-lane motorway to be completed east of Zagreb was the section between Ivanja Reka and
Lipovljani, immediately to the east of the present day Lipovljani rest area; this was done in 1980. In order to maintain the flow of traffic on the Zagreb–Belgrade road during construction, the southern carriageway was completed first, and then the two-way traffic was rerouted to the new carriageway while the old, northern carriageway was widened and its pavement structure was replaced with a new one. Similarly, as the new carriageway extended further eastward, the at-grade intersections were replaced by interchanges or flyovers. In 1985, the motorway was extended by to
Okučani, and in 1986 by another to
Prvča, near
Nova Gradiška. Two further extensions of the motorway happened in 1988 and 1989—those were a extension to
Brodski Stupnik and another one to Slavonski Brod and the present-day Slavonski Brod zapad (west) interchange. In 1991, the motorway reached beyond Slavonski Brod as it was extended once more by , to the Slavonski Brod istok (east) interchange. On December 21, 1994, negotiations led to the reopening of that section for transit traffic. However, the section remained unsafe for use as intermittent violence continued until May 1995, when the area was
overrun by the Croatian army. Inspection of the motorway section carried out at the time revealed damage to a viaduct carrying the motorway over the Zagreb–Sisak–Vinkovci railroad and a local road near Novska. Besides the overall lack of maintenance during the period, the pavement and a number of flyovers were damaged by artillery bombardments, and the flyover approach embankments were damaged where
trenches were dug.
Postwar reconstruction and completion After the war, construction and reconstruction work resumed on the A3 motorway. The battle damage was the first to be addressed, along with removal of the trenches excavated in the flyover embankments. In 1996, the motorway was extended for the first time after the war. This extension reached to
Oprisavci, west of the present-day Sredanci interchange. In 1999, another section was completed, stretching to
Velika Kopanica. The single westward extension of the motorway came about in 2002 as the A3 motorway was extended from Jankomir westward to the Bregana border crossing into Slovenia. while the easternmost section between Županja and the Bajakovo border crossing into Serbia was opened in 2006, thus completing the six-lane A3 motorway. The D4 state road was thus entirely replaced by the A3 motorway. The total investment value of the A3 motorway construction is estimated at 7 billion Croatian kuna (approximately 958.9 million euro), making the A3 the least expensive motorway in Croatia. == Further construction ==