Traffic Question

How is it that you can be cruising along at 65mph on I95 and then suddenly traffic comes to a complete stop. After crawling along for a minute or so, all of sudden the road opens up and you are back up at 65.  Somewhere in the haze of my formal education, I think I remember a statistical/mathematical theory that covers this phenomenon. But it is just another one of those increasing number of things that I find incomprehensible.