I've never seen anyone do that in the UK. It would be illegal anyway but people don't even speed up and slow down. Which countries are you talking about?
Speed on a long stretch between boots, go to the gas station. Average speed is ok, still quicker at the destination.
Can't particularly recommend it, but it does happen. Pro-tip: bring enough cash to pay the fine, because the police will drive you (slowly) to an ATM if you don't have enough to pay the fine.
Honestly I'm completely in favour of both speed cameras and moving average zones, the stuff I see on a week to week basis is shocking, same with red light cameras, I wish every major to minor junction had them.