Tunnel ventilating system

A tunnel ventilating system for ventilating a highway tunnel (2) comprises a plurality of jet fans (6a, b, c, d) for drawing fresh air through the opposite portals of the highway tunnel into the highway tunnel and for causing the fresh air to flow toward a ventilating shaft (1), a plurality of ventilating fans (4a, b, c) for discharging the air in the highway tunnel through the ventilating shaft, and a controller (12) capable of determining necessary rate of ventilation on the basis of measured data representing the degree of contamination of the air in the highway tunnel and other factors indicating the conditions of the interior of the highway tunnel. The jet fans and the ventilating fans are assigned to first and second subsystems. The jet fans and the ventilating fans of the first subsystem are operated under on-off control mode, while the jet fans and the ventilating fans of the second subsystem are operated under variable rate control mode. Thus, the highway tunnel is always ventilated at the necessary rate of ventilation.