The Saltash tunnel is being closed overnight for maintenance checks during the weekend.
The Highways Agency has scheduled the maintenance work from 1900 BST when traffic flows are at their lowest. It will reopen at 0600 BST.
Walls are being washed, signs and lights will be checked and a new fibre optic cable is to be installed.
Work is taking place for three nights from Friday. Diversions will be place for motorists on the A38.
About 30,000 vehicles use the tunnel every day.