What causes slowdowns on the interstate?

This week I attended a talk which talked about flow rate and traffic slowdowns. The speaker mentioned that when the number of cars per hour traveling on a highway reached a certain level, we find that sometimes the traffic will be able to happily keep moving at the speed limit, but usually it cannot. I found Figure 1 in this article which shows the phenomenon. A couple other things that can contribute to a traffic slowdown include:

• Drivers needing to make a decision.
• An existing slowdown. In this old link the author experimentally found that he could reduce the amount of backed up traffic by driving slower but at a constant speed.

Avoiding traffic jams is then about ensuring your traffic network has enough capacity, minimizing decision points, and helping recover from a slowdown if it happens.