There is a black warning sign at the exit of the expressway tunnel. There is a black 80 on the warning sign. What do the black lines above mean?

It is a sign to cancel the speed limit, indicating that the original speed limit of 80 km/h in the tunnel will be cancelled after the tunnel. There is also a sign at the entrance of the tunnel with a speed limit of 80, which is a red circle. The number in the middle is 80, but there is no red line, indicating that the speed limit in the tunnel is 80 km/h, and the speed limit will be lifted after leaving the tunnel.

See the speed limit sign below.