A £1m safety scheme is planned for the toll booth areas of the Dartford River Crossing in Kent and Essex.
The work is scheduled for between 2030 GMT and 0500 GMT
It is hoped a series of new crash barriers will give better protection to drivers using the crossing, as well as workers inside the toll booths.
The Highways Agency said enough booths would stay open to keep traffic moving.
But campaigners say money should be spent on ending tolls while the government claims tolls control traffic and improve safety.
In response to the planned improvements, John McGoldrick, of the National Alliance Against Tolls, said: "Tolls interrupt and delay traffic.
"In America, the toll plazas are the most common place for accidents on the freeways, as drivers have to slow down, possibly switch lanes and fumble for the necessary change.
"Instead of spending money to protect the booths, the government should be scrapping the tolls."
Last month, transport minister Steve Ladyman said the tolls must stay to ensure safety.
"One of the purposes of tolling is that cars are effectively metered as they cross the bridge so it does not exceed its design capacity - it's a safety thing and that's perfectly legal," he said.
Work on the scheme - which will take place overnight - is scheduled to last for around two-and-a-half months from 16 January.
The work includes putting larger concrete barriers at the toll booths and steel crash-cushion barriers on the approach to them.
Booths will have steel frames, while columns which hold up the toll plaza roofs will get safety barriers added.