Average speed check zones. Multiple cameras that identify your car and ensure your average speed between point to point is not above the legal limit. Sure, you can go above the speed limit and then slow down, but it would be a pain to have to slow down, specially if you put cameras the entrance and exit of service stations in highways to discount breaks.
This would be a crazy revenue boost for metro areas outside of rush hour, where interstates are typically 55mph zones, while the traffic collectively moves at 65-70mph; going any slower than the flow is dangerous, but it's a dilemma to risk a ticket or risk a rear-ender.
Not OP but limit cars sold in a country to whatever the speed on the freeway is. e.g. in the UK, all cars registered here must have limiters fitted to stop it exceeding 70mph.
This would prevent speeding (but stop overtaking) on motorways, which I argue are the roads that need the least speed control. Speeding is probably more harmful on all other roads where you have pedestrians, cyclists and complex road layouts.
Oh sorry - to be clear, I was being a little pedantic about he question the parent mentioned - what is his solution to stopping speeding. I agree that reducing Speed limits (particularly on motorways) isn’t the answer. There are other reasons for it however, eg Ireland [0] is attempting to reduce the speed for environmental reasons.