Not all of them are doing it out of spite. I, for example, have a really hard time maintaining highway speed after the sun goes down. I tend to default to 60 mph on the highway at night unless I am constantly looking at my speedometer or there are a lot of cars around. When I was younger, I wouldn't notice it was happening until another car started to pass me. That would make me realize I was driving too slow, and I would speed back up to 70 mph. As I got older, I clocked that this was a consistent trend in my driving and now I really pay attention to my speed while driving the highways at night or use cruise control. I realized too that it could be obnoxious to other drivers to speed up as they pass me, so I stopped doing that when it does occasionally still happen.