In 2008 the City of Stockholm Traffic Administration conducted a series of full scale tests, with heat and smoke generation equivalent to that from a fire in a small car, in one of their urban tunnels with transverse ventilation. The primary goal of the tests was to verify how the ventilation system worked and also to provide an opportunity for the local fire brigade and consultants in this field to see what one could expect in a similar situation. A total of four different tests were carried out at two different locations in the tunnel. Methanol was chosen as fuel and the smoke was produced from smoke machines. The ventilation conditions were set according to a pre-set function to simulate both low and high traffic conditions and the fire ventilation was activated 10 minutes after ignition of the simulated car fire. The result of the tests illustrated a variety of phenomena where the external conditions and the general design of the tunnel had a significant impact on the air flow. These aspects determined the direction of airflow in the tunnel regardless of the pre-set ventilation settings. The ventilation settings for the different traffic cases had a significant impact on the stratification of smoke in the event of a fire.