Oh, that drives me bananas as well! I grew up in Homestead, FL, which, along with Florida City, is the last chunk of civilization before you get to "the stretch", which traverses the last bit of the mainland before getting to the Florida Keys. Most of the stretch is a two lane road (one lane in each direction), but there are occasional passing zones. People will drive 30mph for miles then all of a sudden speed up to 70 or 80 in the passing zones, then slow right back down when the road narrows again. It's almost like they think "well, it's a passing zone, so there's not much to see, so I'll just speed up". It's one of the deadliest stretches of road in the USA, and this is one of the primary reasons.
I used to have a bumper sticker that read "When I grow old I'm moving north and driving slow."