The pile-up occurred between junctions nine and 10 northbound in Oxfordshire on 14 February 2015.
Highways England claimed the move would improve driver safety in the area, though the sensor is not expected to be installed until after April 2017.
Edmund King from the AA said he was "disappointed" it was taking "so long".
He said the stretch of the M40 was described by many as "Fog Alley", and was an area "prone to pretty heavy fog".
Robert Pilott, 65, of Woking, Surrey, died in the accident in which six other people were seriously injured and 55 had minor injuries.
At an inquest after the crash Oxfordshire coroner Darren Salter said he would write to Highways England and ask it to investigate installing fog sensors.
Former paramedic Graham White, from Kingham, said problems with fog there were known many years ago when he started training.
He added: "It was known before the motorway opened, that certainly on the section running down to the River Cherwell and the canal, fog would be a problem.
"We're coming up to that same time of year againâ€¦ when we're going to get early morning mist and fog over the rivers.
"I would've said get [the sensor] upâ€¦ the costs of doing it is going to be way less than the costs of someone dying."
A Highways England spokesperson said: "We are committed to installing a new fog sensor on the M40 near junction 9, which we believe will improve driver safety."
The work is expected to take place during the 2017/2018 financial year.
Two years on from a fatal crash involving more than 30 cars on the M40, Highways England has said it will install a fog sensor in the area.