Ventilation systems play a key role in providing safe tunnel conditions during their normal operation as well as in the case of emergencies. Longitudinal ventilation systems based on jet fans are widely used to develop an airflow to maintain suitable air quality. The turbulent flow generated by fans has a complicated structure because of intensive mixing within the fluid, leading to significant difficulties in its description and analysis. Tunnel areas in the vicinity of the operating fans, where their inlets and outlets additionally affect the flow pattern, are especially challenging. The objective of this study is to develop a concept for assessing the impact of the running jet fans on the airflow distribution from a safety point of view when a hazardous substance is present in their vicinity in the tunnel during an emergency. The experimental part of this research is based on discretising the tunnel cross-sectional zones into a mesh in which a velocity and flow direction are recorded at the cell centres. Preliminary tests confirmed the flow complexity that was characterised by a substantial range of air stream velocities and directional changes. The results provide the foundation for subsequent CFD model development to simulate the flow and analyse in more detail the processes occurring with hazardous substances found in the tunnel area near the jet fans.