The switch-off will also cut down light pollution, the Highways Agency said
Lighting on the M4 near Bristol is to be switched off overnight to cut carbon emissions and save money.
Part of the motorway between junctions 21 and 22 will be left in darkness from midnight until 0500 GMT from 26 March.
The actual junctions, which join the M48 (Chepstow)and M49 (Avonmouth) respectively, will remain lit.
A Highways Agency spokeswoman said the stretch had an excellent safety record and very little traffic during the early hours of the morning.
Chief highway engineer Ginny Clarke said: "We are looking for ways to reduce the carbon footprint of operating the motorway network and this is one step in that direction.
"We expect up to 40% savings in carbon emissions and energy use for each section of motorway where we do this and local communities will benefit from reduced light pollution of the night sky."
The switch-off will be timed, but the agency's regional control centre for the South West can override the mechanism if needed.