The resilience of dangerous goods (DG) transport systems in urban areas has become a critical concern due to the potential for disproportionate impacts on public safety, the environment, and economic stability. Despite the growing body of research on DG transport, the structural concentration of flows remains underexplored. This paper introduces the Herfindahl–Hirschman Index (HHI) as a novel metric for assessing systemic vulnerability in DG transport. The empirical study is based on a roadside observation survey conducted in Novi Sad in 2025, which recorded 326 DG-marked vehicles at eight external cordon locations. The analysis applies HHI across one-, two-, and three-dimensional variants, covering UN code, time interval, and location categories. Results indicate strong concentration by UN code, with diesel (UN 1202) and gasoline (UN 1203) comprising over 80% of shipments. Temporal concentration is moderate but peaks during business hours, while three-dimensional analysis shows overall dispersion with hotspots where fuel flows through key corridors dominate systemic exposure. The findings highlight the structural dependence of the system on a narrow set of fuels and the clustering of flows in critical corridors and time windows. These vulnerabilities emphasize the need for targeted, time-sensitive, and corridor-specific resilience strategies to mitigate risks associated with DG transport in urban areas.