The reason is often because what you think is the speed of the traffic is actually the speed of the asshole in front of you that's going much slower than he could on a congested route.
It doesn't take a single car to reduce the speed of a lane. It takes exactly two. One to slow it down, and the other one to stop the others from letting the guy in the front know that he is going too slow.
I’m aware what speed of traffic means. The actual reason for the phenomenon I described is that there are a lot of hyper-aggressive low-skilled drivers out there. Note that “skill” in this context means driving in a way that optimizes for keeping yourself and your fellow commuters alive so we can all go home to our families; not an ability to weave through traffic and teach slow drivers a thing or two about blocking lanes.
