The cameras, fixed onto tall yellow poles, were put up near to entry and exit slip roads between Portsmouth and Southampton at the start of December.
Highways England has said the cameras will be switched on on January 7, when the smart motorway roadworks get underway.
Narrow lanes will be used while work is being carried, meaning the speed limit will have to be lowered from 70mph.
There will some night-time lane closures in place over the next week while preparation work is carried out.
Average speed cameras work by tracking the speed of your car between two points.
So slowing down to go through the camera and then speeding up will not work. Instead, the cameras take your number plate when you pass through the first camera, then your number plate again at the second, and perform a quick calculation based on the current time to work out how long it took you to travel between the two points.
If the time it took you to travel is quicker than could be done at the speed limit, you’ll get a fine in the post.
So if the speed limit is 30mph and you drove at 35mph for 30 seconds, you’d need to drive at 25mph for 30 seconds to balance it out.