Temporary average speed cameras and a restriction of 50mph have been in place for a couple of months.
Highways England has told Hillingdon is Here the cameras will remain there until mid-October.
The devices are stationed close to the Swakeleys roundabout junction, going to and from south Buckinghamshire, and to the M25 turn off.
Some motorists had told Hillingdon is Here that it was unclear if the cameras were the standard variety speed checks or monitoring the average a driver does on that stretch.
Highways England confirmed they are average speed cameras.
In a statement, the agency said: “We are currently carrying maintenance work along the M40 between junctions 1 and 1A near Denham to improve journeys by carrying out lighting maintenance and replacing safety barriers.
“The work will continue until mid-October. A temporary 50mph speed limit is in place 24/7 during the work.”
Highways England said the temporary 50mph speed limit is in place for “safety reasons while work is ongoing and the safety barriers are incomplete.”
Work on the lighting maintenance and replacement of safety barriers along the main carriageway will be completed by the end of September, officials said, but HA staff will also be working on the slip roads at this location until mid-October.
Temporary signs have been installed at each entry point of the M40 to advise road users that the section of road is operating under a reduced speed limit of 50mph and that the speed limit is monitored by average speed cameras, officials added.
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