External costs of transport, also known as marginal social costs, refer to the negative impacts of transport on society. The user of the transport system, who caused them, does not bear these costs. The primary categories of these costs include traffic accidents, local and global air pollution. Noise and congestion can also play a significant role, especially in specific circumstances, such as urban traffic. In contrast, internal costs of transport are fully covered by the individual user of the transport system. This includes the costs of purchasing and maintaining a vehicle, fuel, tolls, taxes, time spent, etc. Total transport social costs encompass both external and internal costs (EEA, 2002). For the purpose of comparability with other countries, external transport costs are expressed in millions of euros and as a share of GDP. The indicator provides an assessment of external costs in Slovenia for 2021.