A Methodology for Assessment of Visibility during Road Tunnel Fires

Abstract

This article presents a methodology for evaluating life safety risks due to lack of visibility in road tunnel fires. The methodology considers three realistic fire scenarios that involve smoke spread, occupant evacuation, and the reliability of fire suppression systems. The three fire scenarios represent three variations of fire growth rates. The road tunnel, in this study, is assumed to have two traffic tubes, a longitudinal ventilation system, and emergency exits and escape routes. The Fire Dynamics Simulator (FDS) software is used to model smoke spread and the time available for evacuation, based on a visibility limit when occupants cannot find their way to the emergency exits. A simple evacuation model is used to estimate the time required for occupants upwind of the fire to egress through the emergency exits. Vehicles downwind of the fire are assumed to continue moving and exit the tunnel safely. In two of the three fire scenarios, conditions in the tunnel become untenable rapidly, not allowing sufficient time for the occupants to egress safely. A simple risk assessment is used to estimate the number of fatalities, based on the number of occupants who are trapped in untenable conditions in the tunnel. The assessment indicates that a reliable fire suppression system could prevent heavy casualties in a tunnel fire.