The dissertation provides an argument based and systematic overview of the formation and significance of the Croatian highland threshold as one of the most significant and most frequent transport corridors from the Southeastern European continental area towards the Mediterranean Sea. The contemporary geotransport position of Croatia in the European space and the directions of the Pan-European transport corridor are examined, along with the significance of the Croatian highland threshold in that network. Comprehensive analysis provides an overview of the development and significance of transport flows over the highland threshold from prehistoric connections, Roman roads and medieval caravan trails, up to the construction of the first modern gravel roads. Vast material from the literature and from archives and other sources was collected, and field studies were conducted to examine the planning, design, construction and significance of the Karolina, Jozefina and Lujziana roads in the 18th and 19th century. The emergence and significance of other roads in the region are examined, in addition to the development of river transport on the Sava and Kupa Rivers, and maritime transport in the ports of Rijeka, Bakar and Senj. The significance of railway lines for the development of road transport is examined in the transformation of the highland threshold in the late 19th century. The first modern gravel roads were a key factor in valorising the Croatian highland area, and in all forms of transforming the landscape and society. In particular, the influences on the spread of the population structure in the phase of creating a civil society, i.e. in the processes of de-agrarianism and urbanisation. The first roads had a massive impact on the transformation of the economy, with more rapid growth of crafts, manufacturing and industry in the settlements along the roads. The influence of the development of transport on the spatial distribution of the population, and the tourism valorisation of this area are also addressed. The importance of the development of GIS-based applications and georeferencing videos, and the course of development, field planning of route mapping and creating georeferencing videos, is explained in the context of contemporary revalorization of the first gravel roads towards the sea.