This thesis presents the first historical study of twentieth-century road construction and maintenance on a global scale, and the first comparative study of roads outside Europe and the United States (US). From 1930 to 1970, the roads of Latin America, Africa and Asia experienced considerable length increase and, especially in the post-1945 period, the modification of their material characteristics. This transformation enabled the road structures and surfaces to withstand the rapidly increasing heavy traffic of a globalising and nationalising world. This thesis shows the importance of unpaved roads and freight transport, particularly in the developing world. It argues that many roads, especially in the developing world, were built as what were called low-cost roads. Built down to a price, rather than up to a standard, low-cost roads were designed to last half as long as high-quality roads. However, these roads were meant to be upgraded over time, therefore making maintenance and improvements essential to ensure future circulation. In fact, this thesis shows that maintenance and upgradeability were the crucial factors that allowed low-cost roads to bear the rapidly growing heavy traffic of the post-1945 period. This thesis therefore represents a shift away from the main themes of the existing historiography, which focus on private automobiles, motorways, tourism and leisure, and policy for road building. This thesis makes the case for studying the materiality of twentieth-century roads. Road length alone did not guarantee the circulation of rising motor traffic: to talk holistically about roads is to talk about maintenance. Studying the physical properties of roads tells us not only about the link between maintenance, upgrades and the changing load-bearing capacity of roads, but also about the emergence of new road construction and maintenance tools and techniques that originated in the US and spread across the world. Indeed, adding to our understanding of the international influence of the US in the twentieth century, this thesis shows that American road-related standards, methods and equipment were the predominant model for the construction and maintenance of low-cost roads in the developing world, from the 1930s onwards, but especially during and after the Second World War. Professional networks of engineers, and their international exchanges, were crucial for the circulation and adaptation of this model to various environmental and traffic conditions across the world. This thesis focuses on the cases of the Algerian Sahara, French West Africa, Argentina and Colombia – allowing for a comparison of varied extreme natural environments and complex relationships between rail and road transport. Yet, this thesis also discusses the work of American engineers in the US and abroad in the context of the Second World War, and the approaches of British and French engineers in their empires. This thesis emphasises the period after the Second World War, when the particularly rapidly expanding traffic of the developing world accelerated road development and the modernisation of road construction and maintenance processes.