I see this often here in my country, they mostly do it with busses and trucks on the highway as slip-streaming to save fuel, following behind the truck in the wet is stupid, the truck kicks up all the water and dirt from the road, which is worse. Oh right, it's dangerous too (obviously).
We don't really bother with them because if they think life is that cheap to toy around to save a dollar of fuel, then it probably is not worth the time to bother telling them not to. Sounds rather bad but everyone has a brain, can't fault them for not wanting to use it.