Variable speed limits on motorways should be a feature.

It could be a tick box to add (as a speed camera is), which lets users know the maximum speed limit, but also that it is variable in that section.
And/or, users can report what the variable speed limit is while in a variable speed limit zone. A pop up could come up like for petrol station prices.

With enough responses, the most chosen speed limit can be shown to the users as the variable speed limit, and/or used to indicate whether the app will alert the user with sound for an upcoming speed camera.
A lot of times I find myself not being alerted of an upcoming speed camera, because Waze thinks I'm not close enough to speed limit, when in fact the variable speed limit is 10 or 20 mph less than that.