SWITZERLAND'S St Gotthard tunnel was operating close to capacity before last week's disaster.
At peak, tunnel traffic approaches a capacity of 1,800 vehicles an hour, according to the Swiss Federal Roads Office.
Current peak traffic levels are 900 vehicles an hour travelling south and 850 vehicles an hour travelling north.
A new, higher capacity ventilation system was being installed when last week's fire broke out.
Before construction began in 1969, the St Gotthard Tunnel Construction Board set a standard traffic density for the tunnel at 1,800 private car units per hour.
'The capacity of the ventilating equipment corresponds almost exactly to the traffic capacity of the two lane tunnel cross section with traffic in opposite directions, ' says a paper published by the tunnel's design consultants, G Lombardi and Electro-Watt Engineering.
At the time, peak capacity was only expected to be reached for 30 hours per year.
The existing ventilation system, powered by a 25,000kW system of fans, is housed in a void between the tunnel crown and the 120mm concrete ceiling slab above the roadway. Air is pumped into and expelled from the tunnel via four shafts, and at each end.
The void in the tunnel ceiling is split in two, with one half carrying fresh air and the other exhaust fumes. The fresh air is pushed into the tunnel via ducts in the tunnel wall with openings just above road level.
Exhaust fumes are extracted via vents in the ceiling slab. The system was designed with fire safety in mind. 'The cross ventilation system offers the advantage that, due to intensive suction, the fumes can be removed from the traffic section over a length of a few hundred metres in both directions from the fire, ' says the Lombardi/ElectroWatt paper.
Positioning of the ventilation shafts had a major impact on tunnel alignment. The route of the tunnel was placed on a curve, taking it away from as many mountain peaks as possible while keeping ventilation shafts as shallow as possible.
The new, higher capacity ventilation system was being fitted to combat poor visibility in certain weather conditions.
This will also have larger ceiling vents capable of extracting more fumes in the event of a fire.
Installation of the new system resulted from a review of tunnel safety following the Mont Blanc tunnel fire in 1999.
Progress has been slow due to restrictions on work in the busy tunnel and it is now expected to be completed before the tunnel reopens.
In its first year of operation in 1981 the tunnel - part of the A2 national highway - carried just 2.2M vehicles.
By 1999 that figure had rocketed to 6.3M vehicles and was rising.
The 16.3km tunnel took 11 years to construct using drill and blast techniques to get through the granite, gneiss and schist rock, and for 20 years remained longest tunnel in the world.