See for yourself why. There is a river running over the tunnel and water leaks through in some areas. When the temperature reaches nearly 0°C like it does during the winter in Russia, the road freezes and becomes a sheet of black ice.
The result is the attached video which was taken during asingle day with the tunnel surveillance camera. Congratulations to the dual-carriage bus driver -- imagine the passengers in the back! What a ride!
Thanks to Mike!