The author shows with pooled OLS estimations based on transport margins from international social accounting data that investments in improved road infra¬structure have the potential to significantly reduce transport costs. However, this result can only be clearly confirmed for industrial countries and is of primary importance for production and transportation of agricultural goods. For developing and transition countries, in contrast, the author finds other determinants such as weather conditions to be more important in determining transport costs. A key variable, especially in these countries, is corruption. Very high corruption has the potential to prevent positive effects from road infrastructure on transport costs or to even reverse them. This paper contributes to the literature on infrastructure investment by introducing and applying an internationally comparable measure of transport costs which can be calculated for a large and growing number of countries. The author concludes that investments in transport infrastructure can have substantial positive effects especially on agricultural production and the efficient marketing of agricultural products but only if specific additional conditions are given.