Density

When density is high enough our natural reaction to the closeness of all those vehicles behind us is to slow down. This reduces the flow and speed and increases the density , which can cause even more slowness. Eventually, the whole thing grinds down to a low speed. Density is increased by people joining a road from on-ramps, and this causes vehicles to slow down, therefore people are generally bad at merging speed. Then some people who have been held up in the left-hand lane move to the middle lane which causes the same problem in the middle lane - traffic backs up. Then some people dive from the middle lane into the overtaking lane and that slows down.