That's quite dangerous 80 km/h on these roads Very slow indeed. Here the minimal speed on these roads is 70 km/h (120 max) but usually the slowest lane goes at 90-100 at least.
Some areas are limited to 90 km/h. There is a long bridge in the north of Brussels where the speed is limited to 90 for cars and 70 for trucks: sometimes if you do "only" 90 rather than 100-120, you have trucks behind you that flash their lights because they find that you drive too slowly