Many materials are transported by road which, although recognized as flammable, would not normally be regarded as high dangerous (e. g. high flash point liquids, timber). If a fire developed in such materials in a tunnel, however, it could quickly make the ventilation ccurrent passing through the fire irrespirable to the people downwind. Paper introduces a parameter defining the severity of a tunnel fire, analyzes the effect of a fire on the tunnel environment, as well as the factors determining the size of a tunnel fire.