You've been behind the same car for the last 10 minutes, and the driver has been dawdling away, far below the speed limit. You're looking forward to the next passing lane, so you can overtake safely. But when you get to the overtaking lane... the car in front speeds up! It's now going as fast as you'd like to go, so overtaking would mean speeding. You decide to stay behind and not pass, happy that the driver has woken up a little and won't be holding everyone up any longer. Except... the passing lane ends, and he goes 20kms slower again, dropping back to his original speed!
We can all relate to this scenario. Why does it occur? Researchers have determined that the reason is actually pretty straightforward. It all has to do with perception of risk: The wider road makes us feel safer, so we speed up. We also use noise to help us estimate how smooth and safe a road surface is, so when a gravel road gives us noisy feedback, we slow down. Passing lanes are nice and smooth, so they're extra safe when we drive fast on them. But the quiet, and the nice wide roads, combine to provide us with the feeling of utmost safety, so we speed up even when we're not passing anyone - hampering those who might be trying to!
For more on this phenomenon, check out this article.